Under the keys or behind the keys describes where the falling breadcrumbs come to rest. You would use these only if you had clear acrylic keys, or had dismantled your piano to clean the breadcrumbs out, and could see where in fact the breadcrumbs ended up.
In cases where we cannot perceive (or don't care about) the final resting place of a falling object, we generally describe the most important point on its trajectory: It fell through the window or She fell down the rabbit-hole. In the case of breadcrumbs on a piano or laptop keyboard, you would probably say:
Breadcrumbs can fall between the keys.
It's a bit trickier with the keyboard you show, where the keys poke up through a whatchamacallit. (I don't know what it is, and neither do 999 out of 1,000 people; and that's relevant because that means you don't have the option of saying “between the keys and the X”.) But since in this case under, behind and between are all equally objectionable, because they all may be taken to designate the surface of the whatchamacallit, I think most speakers would fall back on the analogy with a piano keyboard and use between, or possibly down between ...
Unless, of course, you made them stop and think about it. There's no telling what would emerge from that.