It sounds like the normal shedding that acompanies a growing turtle. I do not have any full grown turtles [9 -12in] so I am not sure if it slows down or stops when that reach that size, but all of my guys [& gals] seem to have at least 1 scute [one plate of the shell] coming loose.
I would only be concrned if your turtles had soft shells, cuts, or abnormally large pieces peeling/moving. At the present however, everything you described is normal.
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Joshua M Guillemette
2 Children [Deven 10 & Hailey due June 1]
3 Cats [Micah, Judo, & Elizabeth]
1 Bunny [Bugs]
1 Red Belly Slider [Bernulli]
2 Yellow Belly Sliders [Maxwell & LaPlace]
2 Cumberlands [Pascal & Joule]
4 Red Ear Sliders [Kepler, Napier, Gauss, & Kelvin]