sta-rite system 3 sand in pool

Hi guys
I have been getting small amounts of sand collecting on the steps and corners of the pool for the last year or more. The pool and house are new to us. The sand has not been changed for many years I'm sure so I tackled it today expecting to find at least one broken lateral. After removing all the sand, cleaning and removing all 16 laterals I found none damaged. Now I'm stuck wondering how the very fine sand is getting back in the pool? Actually, now that I think about it, the sand particles in the pool seem to be smaller and finer then the sand I scooped out of the filter. The only thing damaged I can see is the little screen on the top vent tube in the top of the filter, it is ripped.