Dead mouse????????? found in babies bedroom?

ello today me and my boyfriend when in to our baby,s room to find the 2 cats we have got thay are indoor cats we live in a block of flats on the second floor ? and when we went in there we found a dead baby mouse? we pick it up and put it in the bin and we like to find out were it come from to see if there was any holes but there was not we do not know how the dead mouse ended up there and why it was dead do you ppl have any answers many thanks