I began to notice some air bubbles in the return little by little, more when the solar panels were in full use so we suspected a leak in a hose. After inspecting hoses from top to bottom, we found no holes anywhere. We have had solar panels on our various pools for 10 years and have always used a diverter system. One of the male fittings on the diverter had a hairline crack that was large enough to suck air, but apparently not to do anything except drip slowly. We all know thats how bad leaks start - nightmare pool emptying episodes can start. I would guess the pressure and chemicals took their toll and possibly some blunt force at one time or another from all the changing out over the years just finally caused the failure.
Thank goodness we checked and found that AND thank goodness I am a packrat and keep everything. I had extra male fittings that were the perfect size from something else. We checked all the other fittings for aging damage and found nothing yet, but my guess is they should all be replaced sometime soon to avoid the eventual. We check all the fittings on the pool itself every year at startup and closing AND the solar panels but forgot all about the diverter!