AlanM
If you're referring to what I think you are, then youmay have a point about the construction of the aircraft and where the transponder aerial is sited having an effect upon their conspicuity to radar.
However, to use a radar head which is miles away when controlling at relatively low level and when you have other radars available is also tempting fate...
To answer the original question: if ATC have 'filtered out' the primary returns (as they may do if there is a lot of clutter), then you are invisible.