Field Mice and Guinea Pigs ?

I have 2 Female Guineapigs and on about 3 occasions i have seen a field mouse in their hutch. I was just wondering if they would be harmfull in anyway. I know its a silly question but i was just wondering. They seem to get along i think; they never sem distressted after the mouse has been in and is alright for the mouse to sit on/ inside the food bowl ?