I bought my husband one of those mushroom kits in a box a few years ago - with portabellos in it. He grew the mushrooms in his basement, enjoyed it thoroughly, and then when they were all done, he dumped the dirt in the side yard underneath a bunch of trees.
Well, every year, right about this time, we realize, "Holy Cow! There's a ton of mushrooms down there!" They keep coming back! By the time we notice them, they're usually almost finished. I finally made a note in our Google calendar to start looking at the beginning of October! We've definitely gotten our money's worth from that kit!

Wow --- if that could be recreated, it would make it totally worthwhile! My understanding was that portabellos and button mushrooms had to be grown on purified manure. Any chance you've got livestock roaming around under those trees?

I don't know what the trees are shading that strip, but they're really common in our area (urban Portland, OR). They drop a lot of leaf litter, so maybe that helps. As far as livestock - several cats roam around there frequently. And that side of the yard is right next to a gas station where several homeless guys go hang out behind to drink and... whatever. So, no telling what all is in that soil. All I know is this is the second or third year we've had the mushrooms come back. And yes, we check them carefully to make sure they're the right kind

I meant "third or fourth year". Sheesh...

Sounds like a unique microclimate. I wonder if your success could be recreated in other high impact areas, like in a chicken run. Or maybe in a graywater treatment area. Hmmm....