I had the same problem with my ferret - I got a Queen sized mattress from my parents (who smoked) and there was a small hole on the side which was from a cigarette burn.
Kodo, my ferret, went and found it the first time I put the mattress in my bedroom and even when I put masking tape over the hole, he still managed to scratch his way in.
End result? Out goes my parents' old mattress and in comes a new one I bought. No holes for Kodo to get into - phew!
