Best I can gather the pool is about 8 years old, house is 9, so there should be no remodel work. The small pipe definitely only connects to the skimmer suction line, I pushed the gravel aside enough to see that. It's made up of a reducing tee in the suction line, the horizontal section, a 90 el, the vertical section, and a cap.The big one might have led to the pool and been capped off after a remodel covered up the corresponding port in the pool.
The small one could have been some sort of manual or auto filler. If you decide to cut that one to see what it might be, you should turn off the water main first. Not just the one to the house, but the one at the street. Sometimes landscaping and pool supply lines are tapped into the water main before they go into the house, so are not controlled by the house's main shut off...
Guess I need to take a closer look. If that is an air inlet, that might explain why I can't seem to turn off the bubbles in the spa completely. I re-lubed the O-rings on what I figured were the air inlets (one per jet), but they still don't seem to seal completely. If there's no air pump for the jets, and the jets blow air bubbles already, what would the purpose of plumbing that back to the pad? The jets in the hot tub have some sort of wobbly massage bits in them, and they seem to work fine, would that be related?It looks like the cap on the big line has a slit cut through it on the back side, if so then its probably an air line for the spa jets.
the ozone unit connection remnants ( tubing that connects to the pump ) should be removed, and a pump plug should take its place, this tubing has the potential to be a bigger problem than it looks, trust me. the 1/2'' line is ozonator connection also, i would hack it off at the tee, and cap it.
most air lines will have a slit cut in the pipe, the cap, and the cap is not supposed to be glued