Happens in certain offices I work in (I'm a consultant).

What I think happens is:

Someone puts a number of items in the dishwasher, some of which wasn't used at all, they came back on a trolley from a meeting.

Next person comes, checks two cups and think it is clean and than put everything in the cupboard.

You might have just eaten something dry, and when you put the dish/bowl in the dishwasher all the crumbs fell off.