FWIW, I had a close encounter with an F-15 a few years ago. Local ATC had been sending inbound military traffic over our class D airspace at 3500 feet. I called local ATC, their response was "if you do not have a transponder I cannot see you". New young controllers rely totally on the technology. We have since had several get meetings and now have a great relationship with them.

Also gliders when circling have a ground speed that is less than 40 mph so the radar software ignores the contact. So "type and altitude unknown" call will not be made.

C5