Some of my collections appeared completely empty when I've opened them!
Apparently they were filtered.
Well, if I was the only person to use them I would have noticed immediately that it is filtered, but since someone else erased the items under that filter - It took me some time (and few missing heart beats) to recognize that.
I think it is necessary to make some indication for such cases. I should know if I'm looking on a filtered table.
Of course, from now on I will check this every time, but it seems better to have some indication without opening the filter menu.