When I leave my mandolin for a few days and come back to it, I always find it slightly sharp on the G, D, and E strings. The A sometimes, but not so often.
The mandolin stays in the same room with no great temperature or humidity variations all year round, AC in the summer and heat in the winter. The thing that puzzles me is this. I can imagine that the strings might be slowly stretching; I can imagine that the tuners might be slowly turning in reverse; I can imagine that the neck might slowly bow or that the top might slowly dish, but all of those things suggest that the instrument should go flat, not sharp. I can't think of any passive process where tension on the strings would increase.
Why do the strings go sharp?