I got a lock on a F117 once whilst we were doing an offensive VID. The USAF weren't too happy when I blurted out the ID of the aircraft on a clear frequency!!!! Apparently at the time it was 'optimised for surface to air threats'.
As others have said, and other countries have done, you can develop techniques to see most things within the EM spectrum; you just need to be sneaky or someone will develop a counter to your counter and then the EW merry-go-round begins.
LJ
