Lets sum it up:
You say PCcv is correct by 50% when he only quotes SD card.
So one half of whatever it is must be a SD card.
It is in a glass like thingy that looks like a bathroom sink or tubs overflow window and you say every body uses it for half of one full day, on daily basis!
That brings us only to one thing and that must a cellphone with an unusual shape or a SD card that is fallen into bathroom sink/tub, showing up against the overflow outlet
.
No ideas at all otherwise unless this is something that happens during that half of every 24 hours that I am in sleep so I just can't recognize it.
Anyone with any ideas, say it loud.