There's a chance one might pass through, which seems very alarming to me. It also seems like we'd have observed interactions between such black holes and other stars pretty frequently.

I don't know if we'd have expected to have seen lensing effects from our whole-sky surveys, but it seems to me that this would also be probable.

Finally, if you send a line through our galaxy, you'd expect it to pass near a lot of these black holes -- and thus you'd expect to see a lot of microlensing effects which the article states don't seem to occur nearly often enough.