Rainbow Six Vegas wasn't actually UE3 either, I think?
R6V was confirmed using UE3. It was an early build, earlier than GeOW I think, with quite some in house adaption, making Ubi the first to actually have a UE3 game.
Before crashing and dying a horrible death and getting restored oh-so-partially, BeyondUnreal's wiki had Scimitar linked with UE2.5 (an adaption from Splinter Cell Double Agent's engine)
Now it's removed. Maybe the folder structure on PC wasn't exactly like UE2.5, but some devs do pack level data into big blocks instead (AC actually utilized that as an antipiracy measure from its early leak in March so it might have been intentional)
[And I do have to say, Scimitar's way of shading, rather low polycount (wrt anything else during the same period), and alpha texture AA quirkiness are REALLY similar to UE2.5.]