I should add that in my experience it's a bit misleading to assume that Red-eared Sliders will always show a distinctive red mark on the head. As with most other ID issues, there is variability in individuals of a species. And a bit of mud on the head or the angle of the light can obscure field marks surprisingly well, too.
I'm not saying they don't have the red mark, just that it's not always obvious, and can be variable, and that you may need a good look or a good angle to be sure it's not there if you don't see it initially.
Also, small immature sliders can show a flatter profile to the carapace.
I haven't worked with turtles, so I don't know the appropriate place to report rarities, but someone who knows please post a link for reporting rare turtles so this can be done if any are photographed.
If anyone knows of any official programs to release Pond turtles, please let us all know.