something tunneling into new rabbit barn

We've been so pleased with our new rabbit quarters in one end of a new barn built this summer. The barn was built with posts placed on foundation piers of concrete 4 feet deep. Then we filled in with rocks from one of the many piles made years ago when fields were being cleared and topped off with gravel (that's what the guys called it, looks like small rocks 1 to 2 inches diameter). Before we moved the rabbits into their area we made sure there were no spaces at the base of the walls where any predators could get in. And thought the rocks would deter tunneling.
Now we've noticed a couple small tunnels under the wall between the goats and the rabbit area. Not very big--I'm thinking rats :x . But I know there are mink and weasel and feral cats and coons and who knows what else out there. The shed we had built to hang our wire cages in last year was tight--all wood, and when the door was shut nothing could get in. I wanted this new space to be tight enough to keep everything out. I think some of the rest of you have rabbits in structures with wood floors. Do critters tunnel in? If the tunnels are small, should I just get over it, recognize that rats and mice just come with a farm with stored hay, spilled grain etc.? Would larger animals, predators to our rabbits, be able to tunnel into that kind of floor? :? Any advice or suggestions?

Our rabbit/goose house has wooden floors and rats chew their way in. We drop poison down the holes and patch with metal. Nothing can get at the poison except the critters who shouldn't be down there anyway.

On one occasion when we did not patch a hole in the floor promptly, a mink found its way in and made itself at home. It killed a large doe and was systematically feeding on it over at least a couple of days. We were lucky that it didn't go on a killing spree, but it only took the one. MidnightCoder saw it one day, chasing the rabbits around the colony, and dispatched it.

There are sprays that are fairly good at repelling critters. Ropel and CritterRidder are a couple that come to mind. We use them when we get a raccoon or skunk hanging around before it figures out how to get into the buildings. Of course, with the pop-holes open for the goose and chickens during the day, sometimes we will still get unwanted visitors bold enough to raid for eggs during daylight hours.