I can't see sand pushing up unless it's an IG pool with a lot of ground water pressure washing sand up through a hole, and that would mean the water pressure under teh pool is greater than the water pressure downwards in the pool, so your water table would be above your pool water line for that to happen.....
Is it possibly atmospheric sand - ie crap that the wind brings?
I get fine silt in my pool all the time from the fields and such, around my area it's clay dust because we do not have sand around us.
If it is in fact a hole, and sand is working it's way into the pool from it, then you should be getting an indentation where the hole is since the sand needs to be replaced by something - and that something would be the liner sinking into the hole left behind. Have you checked your cove all the way around?
Also, the sand probably won't travel too far from the hole under normal circulation, so if you don't run your vac for a few days, see where the sand collects, then it's down to that area as suspect.
Holes usually look like a little speck of dirt (black dot), it could also be a little slit (more likey).
I'm still baffled about it though since I cannot see that happening.
Possibly ants?? I know they carry grains of sand out of their tunnels, but those would have to be mighty good diving ants!