So, it looks like we found where some of the occasional mice are coming in from!
This is a sump pump outlet. On the inside, the pipe is well-sealed. But somehow, the occasional mouse goes in and comes out in the same spot at an adjacent wall, right into a trap.
On the inside, the area around the pipe is well sealed with what looks to be some type of [hydraulic?] cement.
For the outside, I’ve seen conflicting internet advice. 2 magazine-type sources say that it should (could?) be filled with expanding foam, and then covered with roof flashing that is tarred to the foundation surface.
When I found a similar forum thread somewhere, some folks advised against hydraulic cement, since the rigidity could damage the pipe over time.
What would you guys advise?
(Normally, there’s a tall clear-out pipe blocking the view. The soil around this pipe must have been eroded recently, or dug out, as it was well covered over the winter.)