Don't want to throw a scare at 'ya raymr, but another thought could be...
A cracked/split return pipe feeding back into the pool, post-pump.
Back in '68, the step-father failed to bail the return lines properly before winter set in.
Springtime '69, we were seeing trace amounts of sand coming into the pool from the return vents. The pool folks came out, thinking it was the pump/filter arrangement going to hell, but they quickly determined that it was the return line, sucking sand back into the pipe and spewing it into the pool. The other "give-away" for the trouble-shooting of the cracked pipe was the fact that we were adding a "bit" more make-up water than usual.
Here's the "kicker" of the story...I'm the "mole" that ended up digging out the faulty pipes under the concrete deck that spring/summer, cutting and doping the piping back together and "repacking" the sand back into the tunnel/hole to complete the job. 12-years old and I'm a professional pipe-dope sniffer.
GAWD, that summer dragged on...
BTW, not only were the return lines BOTH split, so was the line from the skimmer, too. My mother ended up filing for divorce from the guy before winter set in, so I ended up "closing the pool" for the next couple of years, while we still lived there.
No cracked pipes on my freaking watch!!!! There's no way, in all of H3LL, that I was going to be under the concrete digging out, and pounding, sand for another summer!