Sand

We have not changed the sand in our filter for 3 years. The return flow is very, very strong. Could some of the sand left the building?? Also, we have little dirty pools at the bottom of the pool. Water is clear and chemicals are all good. Thanks for your help in resolving this mystery.

Tells us more about the pool, pump, and filter. What kind of pool, above ground, in ground, liner, plaster, fiberglass. Size, make and model of pump and filter.
Can you open the filter to inspect the sand?
Have you? Do you see filter sand coming out with the backwash?