Teleporting crystals?

Notice that some of my rocks seem to appear and reappear on their own? I keep all mine on my windowsill and will often notice they will be gone one day, only to notice their reappearance a few days-weeks-months later. Straight up AWOL.

Why is this? I've heard this phenomenon over the years, so I know I'm not the only one. EA - can you tune in as to why this happens, and where do they go?