Every now and then after a big rain these crop up in my basement. No obvious cracks or anything where they pop out of the unfinished concrete floor. I try to dispose of them without too much opportunity to shake of spores, and then douse the floor with bleach, but I also admire their ability to grow with no apparent nutrients.
Interesting, the nearest tree was the street tree across the sidewalk at that end of the house, died a few years back and was taken out. Come to think of it, I'm pretty sure there were some roots coming through the stone foundation wall when we moved in (since sealed & insulated). There's a new thriving tree in its place though, I wonder if the mycelium hop to the new roots.