Most likely one from last years brood. They grow slooow.
Best story is my oldest sister commenting that she never sees turtles in SE Michigan. I sent her probably 8 photos in 2 weeks of turtles I had spotted on the road or in my yard. Then told her to look off the transom of her and her husband's boat while it was sitting in the dock. Those green sticks that were poking out of water were turtles. She had to get out the binoculars to prove me wrong.
That didn't work.
She never was the most observant one in the family.
30+ years in the same dock and never noticed.