Hey thanks for all the quick responses guys! I'm currently at work so I'll measure him and try and get some pics up here later tonight.
My temporary answers:
How big is he? The turtle is about 5" long
What is he fed? He is fed good quality turtle pellets and vegetables once a day
Does he move his feet really fast? Yes, constantly swimming against the glass
Does he do this when you are hiding/away? Yes, he doesn't seem to care whether I am there or not.
what happened to the other turtle? Still at my sisters, soon to be together at my house in their new tank!
more info on his setup? it's a 20gallon long tank with good filtration, UVA/UVB bulbs, nice basking platform, plenty of open swimming space and rocks on the other side to rest on. (soon to be in a 55 gallon with the other turtle...Maybe he will be happier there?)
I can see if he was hungry and learned that if he does this someone will feed him but I can't see that being the case. I'm leaning more towards marisa's suggestion that he may be seeing his reflection and thinking it's another turtle. on the other hand he has been doing this for almost 2 years in 3 different tanks, maybe he is just "special"

if you know what i mean.
I'm going to try and put black garbage bags covering the back and sides of the tank to see if that would make any difference to him.
Thanks again everyone! keep ya posted.