my guess is that is came from a seed bank. Seeds can remain dormant in soil for years until conditions are favorable and then they germinate. In some plants seeds can survive for hundreds of years in a dormant state.

Or a bird dropped the seed. Be thankful for the gift from nature!

I have a bunch growing along one of our fence lines,cute

Looks like a nice shade plant. It is beautiful, but likely short-lived. I hope to one day have Mayapple in my shady area. If it (The Trillium) has wild seeded itself, you are lucky, and have a nice area, which you can grow many plants that need a certain type of area to grow in, with "woodsy soil" and likely many old tree stumps, or logs. Even bringing old logs from one area to another, is okay to start some plants. But they need shade. So in other words, if you have some of the special plants, Trilleums, Pitcher plants,Mayapples, etc. You are lucky.

I went back looked we have a garden of jack n pulpits,may apples.NowI know jip.give seeds do mayapple?I would send you some if they do

I have mayapples all over the place and unfortunately, they can get to be an invasive nuisance if they like the location. They grow by root division and also seed which spreads widely. Nice ground-cover in moist areas but they tend to strangle out many more desirables.

As for the T. luteum, I find that they stay in bloom for quite a long time, they are still attractive here having been open for nearly a month, and the foliage is long lasting even though they are generally considered Spring ephemerals.