perimeter foundation? (vents at bottom of siding) cleanout plug(s) may be in crawl space.

cleanouts in the 1972 might have been retrofits...

I saw your tarred water service post. (that rectangular hole in siding looks weird to me..)
you have slab (seems rare in 1980.) slab under garage only?

If all house is slab, I'd guess your only cleanout is coming 90 degrees out of the wall nearest the "downstream" toilet (or the only toilet).
the soil outside that wall might be soft if someone had to find and open the cleanout.
visualize about 5 feet from wall. there might be a mark on the curb, aligned with that offset from house wall. (everything at 90 degrees on lot, rectangular lot, straight streets, etc)
my best guess... :-)

I think that hole is just a broken piece of drywall from the other post. The slab is under the whole house. The street is on a pretty good slope and house is cut into a hill. It is a two story house with 2.5 baths.

developer subdivision? maybe neighbors know how their drains come out of the house, where to dig for cleanout, and yours would be similar.

Per the plumbing codes the line which runs from the house to the municipal sewer or septic tank is defined as the "House Sewer" while the primary drain in the house is defined as the "House Main Drain".

The codes make that distinction because some of the materials that are approved for the house sewer may not be used for the house main drain and vice versa.

Peer code we are to install a "House Main Cleanout" at the junction of the house sewer & Main drain. The cleanout is to be placed within 3' of the point where the house main drain exits the structure.

If the house has a basement the main cleanout is generally in the basement where the pipe enters the structure.

For slab construction the main cleanout is placed 3' outside the footer wall.

And in the case of a raised foundation with a crawlspace the main cleanout may be placed either in the crawlspace or outside the footer wall as done on a slab.

It must be understood that the DWV rough in is completed before the slab or basement floor is poured unless the line will enter the basement at some point above the floor.
When making the rough in for a slab construction it is generally done before the footer wall trench is backfilled so when the plumber installs a cleanout outside he will extend the cleanout up to the theoretical finished grade level, however, in many cases after the trench is backfilled and they begin the landscape phase often the actual finished grade will be higher than what the original theoretical finished grade was proposed to be and this often results in the cleanout being buried by the landscapers.

Thanks everyone. I think even I were to unearth this drain it would not be where I want it anyway for easy draining of the spa. The kitchen sink is closest ... I think I am just going to run PVC thru the wall and make a drain there. Good to know about the main clean out should I ever need to go there. Thanks.

My house built in 72 just had one sticking out of the cement at a convenient location so guess I was spoiled since I didn't have to do anything special for this part. At least the electrical to the SPA is easier to run at the new place which is nice. Straight shot from the box under 100ft away - no cement in the way!