Plugged Return Line Newbie

My wife and I just bought a house with a pool and neither of us have had one before. I've figured out most of it and winterized the pool myself. I'm a DIY person with skills ranging from rebuilding motors to building entertainment centers from scratch. I can typically figure something out on my own or with minimal research.

Anyway. Before winterization I noticed a return to the pool wasn't supplying any water. Back up, It's a 40,000 gallon inground pool made of concrete I think. Kind of shaped like an 8. Also has an attached inground spa with spill over into the pool. The pool has two skimmers and a main drain, spa has two main drains. I have a sand filter, one main pump for the whole system, and valves to shut each skimmer/drain off individually but one valve to turn on the entire pool returns and the same for the spa. Also has a heater.

Ok back to my issue. One return to the pool is jammed good. I hit it with the shop vac both suction and blowing on both sides and I get zero movement. So then I got a rubber plug and drilled a hole in it, used a shop compressor to pressurize the lines to approx 30-40 psi from both ends and get nothing. I even tried pressurizing while having the shop vac sucking the other end. Nothing.

How does a return line get clogged like that and how do I unclog it? Anything that got to the line must have gone through the filter unless the valve was on recirculate. I'm confused and stuck. Please help a pool newbie out.

Also, I'm in Michigan if that means anything.

Update, I had a capped pipe near my filter so I had a hunch and drilled a hole in it. Sure enough it goes to that jet. Why would there be a jet that's piped separate from the rest of the system, and then why would it be capped? I blew it out and capped it again this time on both ends to keep it winterized. Any insight?

I have no idea--I just wanted to pop in and let you know that forum traffic slows way down this time of year, so you will probably get responses, but it may take awhile to get them. Don't give up! :)

Maybe it leaks and the previous owner just retired it in place?

It doesn't leak, I pressure tested it. And it's plumbed separate. All other return lines go through one pipe, into the ground, and split in a manifold somewhere underground. This one return has it's own pipe. Is there any reason you'd want one return to bypass the automatic chlorine feeder? I have a T just before the feeder which is capped off as well and looks like it might line up with this capped pipe.

It's a mystery. The line is good, just capped?