Could there be a ductwork from the furnace in there?

Was a fireplace and chimney or pad for a wood stove removed?

My house isn't that old, but when I remodeled 2 of my bathrooms we were able to increase the size of the showers by using some dead space that had been framed in. No pipes or anything hidden, just somebody deciding where to put a wall.

Ductwork all runs through the ceiling. Or rather, along the tops of the ceiling, UNDER the ceiling and inside the room (rather than up in the attic, though there must be a place in the attic where the furnace connects to the heating ducts) In fact I'm pretty sure that's why they put that stupid "bookshelf" there (with the shelves too close together and NOT MOVABLE eg too short, and too deep for actual books and also not suitable for decorative items either, LOL!). Basically it fills in the space under the ductwork, where in the kitchen (on the other side of the living room wall past the bathroom) the ductwork runs over where the wall cabinets are.

The story of these houses is a little weird, they were supposed to use plans that included WOOD STOVES (wonderful way to heat in the desert where trees don't grow) but instead used plans intended for a different Air Force base in another state that included oil furnaces. Thankfully. But the oil furnace was replaced in the 70s or 80s with a forced air gas heater and that was pretty nonfunctional (wasn't even correctly connected to the ductwork). So we had that replaced and the existing ductwork at that time was all up through the attic and running along the top of the ceiling in every room.

Anyway so originally they intended to build houses that would have included a connection for a wood stove but since they didn't actually USE those plans I'm pretty sure it was purpose-built for the oil furnace originally, since those were the plans they actually (accidentally) used. Maybe there was/is some stuff involving the old oil furnace in there ...

That room is WAY too big for a "coat closet", plus the actual coat closet (also somewhat oversize but still obviously closet-ey) is just around the corner there. The "weird" room is 4 x 6.25 and would be 5x7 ish if it weren't for the weird bookcase and offset back wall with apparently blank space behind it. The coat closet around the corner is extra-deep and larger than any other hall closet I've ever seen @ 38.5" deep - more than 3' deep - and 41" wide. It's HUGE for a closet but at least it actually looks like one.

As for that odd space seemingly blocked off there between the bathroom and the weird room, I guess I'll need to take out a bit of wallboard and look. Just seems so odd ...

I had a 12" void in a wall between a hallway and a bathtub. Not sure why they did this but I'm assuming it was so that the alcove tub was flush with the other wall and not inset more. When redoing the bath I took that wall down and made a large walk in shower. It's strange why people would take away floor space but it does happen.

In my old master bathroom they had created a 6" void with double-framed wall to shrink the size of the shower. Similarly they lowered the shower ceiling about 12". When I remodeled I opened it all up and had a nice larger shower. Looking at the drawing, maybe they did not want the tub to be recessed so far back into the nook?

Yeah, it's weird. I built my own house about 20 years ago so the idea of doing something like that is just mind-boggling to me. As a homeowner I"d rather have the useable space than a weird void in the wall. Plus, all they had to do was wallboard the back of that wall where the tub is, it EXACTLY matches up with the ginormous-for-a-closet-but-too-small-for-an-actual-room space on the other side. Instead they frame in a whole other wall just 6 or 8" from the existing one? LOL!

I surmise that somebody measured once and then didn't want to cut twice, eg somebody got the measurement for the framing wrong and rather than take down the "wrong" wall they just threw up a second wall to make it "right" in the bathroom and counted on nobody noticing it was "wrong" in the closet.

Then somebody with a hypersensitive feel for space comes along and starts wondering why things are off kilter. LOL!

At least I can stop worrying about there being a body walled up in there. If it was even 5 or 6" wider, I'd have to wonder ...

We had an 8” space behind the shower plumbing and assumed that space held pipes or something...but no...it was just an empty framed out box. House was built in the mid 60s. I have no idea why they did this! When my cousin remodeled his 70s house, they found a giant cavity behind the master shower...and were afraid of what (or who) they would find inside it. But it was just an empty, oddball space. They ended up using part of it to expand their shower and the other as a big closet from the hallway.

We bought our house in Baton Rouge from a couple who had been LSU professors. He had just recently died and she was going to move to Washington DC, where he was buried. They had lived through WWII in Europe. He was Polish, she had been born Russian but had escaped the Russian Revolution as a child, her family fleeing as they were part of the aristocracy. Both were caught up in the Nazi horrors and they met in the Displaced Persons camps at the end of the war. They came to America and led comfortable lives eventually, but must never have felt totally secure.

In the wood paneled den, the desk and the bookshelves above it were recessed into the long wall. It looked very nice, all built in. On the other side of the wall, the living room wall did a jog in about the same place, so it was not really too obvious at first that there was a big square footage missing. The kitchen was right next to the den, and the closets in that room kind of tricked the eye into not seeing the missing space, as well.

The first spring, my husband finally went up into the attic and sent down all of the things they had left behind - Christmas trees and ornaments, Time Magazines in multiple languages going back to the 1960s, lots and lots of paper, and all of it covered in cockroach droppings! So much junk! But it was easy to understand that the widow would not be able to clean out the attic herself, and they had no children. As he made his way past all of the stuff, my husband uncovered a space. He almost fell into it. It was a little space just a bit bigger than the cot that was sitting at the bottom of it. A ladder allowed you to climb in and out. Next to the built-in desk, and going into the jog in the living room, there was a hiding spot accessible only through the attic.

Apparently, this couple had learned the lesson well, and were not going to get caught unprepared in tyranny again.

I have known people who survived the Holocaust and they pretty much all had some sort of emotional scarring like that. A lot of it revolved around having plenty of food stored, which was a shared characteristic with people who lived through the Depression (like my parents). And that was actually the kind of thing I first suspected here, a gap of 2' or 3' wide.

Things just didn't fit together right. When it turned out it was only 6" or 8", I didn't quite know whether to be relieved or disappointed! Although when you count the extra framing and wallboard, its closer to 10" or 12" all told. In my own defense, LOL!