I get seedling Iris sometimes, and sometimes they mature before I
notice. I found one a couple of years ago growing out of a quarter inch
crack in a driveway, healthy and blooming beautifully, nothing like any
nearby. I got a piece because I liked it. Good thing, the original
is extinct now.
Not to say that this is what you have, but seems like one possibility to me
Maybe somebody was "dumping" one they didnít want anymore and thought you'd
give it a good home (sort of like people who dump there dog on somebody who
already has dogs?).