Any question answered...

Rat.

First hand experience suggests a good sized one - a 2" hole appeared in a tupperware sandwich box during my first week at work. A 'snap tin' like this:

• was the only way to stop the buggers sharing your lunch.
  
  

Options:
Live with your new found friend
Tame it
Trap / kill it
Report it and let someone else kill it.
sell it to tesco for their burgers

ftfy