You know when you hear something and think "pffft, like that will ever happen to me"? Well I just had one of those moments.
At some point in the last week or so we discovered we had mice in the office. We turned the place upside down looking for them but couldn't find anything. We were baffled where they (or it) could be.
So we get this guy in to get rid of them and he says "They like to nest in warm places" and starts looking around the water cooler.
At this moment I instantly realised there's a blanking plate or two missing from the mail server stuffed under my desk in the far corner. I pull it out and remove the side panel. Sure enough there's mouse poop everywhere. About a second later this tiny mouse runs through one of the blanking plates, sits on it's hind legs, looks me in the face... then jumps up into the drive bays. By the time I'd removed all the wires and pulled the PC out it had vanished again.
So yeah.. might be a good idea to cover any gaps with blankin plates, regardless of how much you might think you're safe