Commonly called smurf tube because Carlon, the most common manufacturer, makes it in a blue color. Officially, it's flexible ENT.

Most likely they didn't actually fit it to the panel. Look around in the wall in the vicinity of the panel. Similar to the fact it just loops into nothingness in the attic, it probably loops into nothingness in the wall in the basement.

Unfortunately the wall that it goes down from the attic is my exterior stucco garage wall and is covered with sheetrock on the inside with all kinds of "stuff" attached to it. No easy way to access the interior cavity from either side of the wall. Perhaps I can run a piece of cable down through the tube until I feel it hit the panel and then try knocking out one of the larger knockouts at the top of the panel to see if I can bring it through. What do you think?

Sure, an have someone jiggle an electrical fish down the tube and perhaps you can find. However, you're not likely to be able to connect it to the panel legally without cutting into the wall but knowing where to cut would minimize the potential damage.

That conduit may already be connected to your panel;. Look for a bushing with a "conduit penny" in it. The "penny" is a blank to close the path from the panel to the open upper end of the conduit.
You can Google the conduit penny for better understanding.

Thanks! I found the bushing with the corrigated tube attached. It didn't have the "penny" in it as you decribed but I was able to insert a wooden skewer about 6" into the tube and by scaping the inside of the tube I could feel the tube's ridges.
One last question. I noticed that the vertical bars on either side of the breakers have both neutral AND grounding wires attached to each. Does that mean that when I run my new circuit into the panel that I can insert the neutral and bare copper conductors into any open spot on either bar?

If this panel contains the main disconnect, then yes the grounds and neutrals can land on the same bars, there's no electrical difference at this point.