So far, I've heard about those in a few instances...
Season 13 - Leg 9 - This one was confirmed by at least one team here and it was claimed to be at the military camp and given the nature of the U-Turn, it is at the military camp.
Season 15 - Leg 3 - This is the murky one as it claims that U-Turn was blind and was used by a team to no effect. There isn't a location associated with it, so where ever it was, that U-Turn was edited out.
Season 15 - Leg 8 (or 10?) - The second of the U-Turns, if it did exist would have been in either of those legs, since the U-Turn normally is right before a roadblock and not right before a pit stop.
Season 17 - Leg 6 or 7? - There is some claim that a (possibly blind?) U-Turn was located on Leg 6 (at Palace Square) - However, I think it may Leg 7 that had a U-Turn since the Detour was up first and both options were at the site itself.