I thought I remembered seeing cloaked Recons as masked IR signatures, light blue blobs as opposed to gray.
Although, if you've ever used IR equipment/goggles and tried to recce a building with them you'd see that any heat signatures behind glass are completely obscured. I suppose that a good cloaking technology would use some kind of glass panels/fiber as an exterior to help block out heat signatures. It wouldn't be undetectable, and footsteps and the like would still leave a subtle heat trail.
Actually, now that I just ranted about IR and heat trails... That would be a cool feature to see when using HEAT. Faint heat emanating from the last few footsteps of all players, including cloaked Recon players.