I'm sure that Lower Airspace Radar Operators can be concerned about gliders.
They don't usually transpond, nor communicate, because their behavior and requirements do not easily fit into normal traffic patterns. They travel in large groups, and behave in eccentric ways.
I am sure the lower airspace Radar chaps understand that this happens only in certain weather (good soaring conditions) and at certain altitudes - usually from 1,500 feet AGL to cloudbase. So predictable and regulated traffic can be warned when these conditions obtain, and where gaggles of gliders may be observed on radar.
On a good soaring day, a while ago, I was over Aylesbury, in my 15 meter single seat glider. Upper Heyford was the nearest LAR operational, so I made my call on the radio...
"Upper Heyford, this is Glider 987."
"Glider 987, squawk 2 4 6 8 ...."
" Sorry, unable. Negative transponder!"
"Glider 987, what is your present heading?"
".....! ! ! I'm going around in circles!"...... Long pause while everyone on frequency must be laughing like drains.
" Glider 987, what are your intentions?"
"Well, I thought I would cross over Upper Heyford to Banbury and then to Husbands Bosworth Gliding Club"
"Notify us when you approach the Heyford zone, and maintain 3,000"
"I'll try!...."