Yes this is his something and normally it isn't a big deal. How often does he find himself needing to venture into any small, dark space? Apparently it is often enough in Mathias that has made him unsettled already.

"I appreciate your kindness, but I do not wish to stop you from looking under the house if that is what you want. Is there anything I could do to help?" If she finds any rabbit holes, he really doesn't want to go with her, but he would. There is no one else here but the two of them. And he would not risk anyone else's life or sanity even if there was. The guilt would be crippling if anything were to happen.