A safe house has to be thoroughly checked out. But I think it was the SPD who owned the house so why would we have expected them to even look under the rug? That would show an iota of intelligence.
BTW, I got the impression they didn’t just hide under there but that it led out of the house and into the woods. I’m sure the blast would have blew out the flooring also and it would have affected them. Or, at the very least, gotten them a little bit dusty/dirty.