You keep mentioning the "inplaces". I assume you mean the skimmers where the water is drawn in.
How about the returns- the jets that blow the water back into the pool? Have you put some sort of trap on those to see if there is sand blowing out of them? Do you have a sand filter? You mentioned covering the pool in winter. If you have a sand filter, it could be some of the laterals cracked due to freezing. The easiest way to tell would be to unscrew the return plate, put a filter bag made out of old nylons across the return, then put the plate back and see if the bag gets sand in it when the filter runs.
If I am not mistaken, scale is white and flaky whereas sand is... well . . . . just sand. But I will gladly defer to Duraleigh's knowledge- it's more extensive than mine.