I don't think we have any glider fields close enough to hear in the Boston area..
When I went to check out the tech details at
http://www.rtl-sdr.com/receiving-decoding-flarm-tracking-gliders-helicopters-etc-using-rtl-sdr/
I saw it looked like 'weak signal work' and would be very difficult in this noisy RF environment.
Then, I saw the link to view gliders being tracked-live..
http://cunimb.fr/live/3D/
Since it's late in the evening here, I figure it's very late in Europe..
But, there were still two gliders on the display! Brave night flyers?? So, I zoomed in on them..
First one in the UK,
Regist.: e5f9b652
Flarm Id: hidden
Type: Glider/MotorGlider
Last time: 02:23:22
Latitude: 51.189499
Longitude: -1.034500
Altitude: 197 m
G.Speed: 0 km/h
Track: 0 °
Vz: –0.1 m/s
Receiver: EGHL
and the second somewhere near Italy.. (Graz airport)
Regist.: 2a06f153
Flarm Id: hidden
Type: Glider/MotorGlider
Last time: 02:22:58
Latitude: 46.996498
Longitude: 15.432000
Altitude: 349 m
G.Speed: 0 km/h
Track: 0 °
Vz: +0 m/s
Receiver: LOWG
Humm, no ground speed.. Zoomed in close.. Both are parked inside hangers at glider fields!

I think a couple of folks might be testing their battery packs..
Edit many hours later:
Glider or radio (at Graz airport) has been moved..
CN: _53
Regist.: 2a06f153
Flarm Id: hidden
Type: Glider/MotorGlider
Last time: 21:02:17
Latitude: 46.157700
Longitude: 15.432000
Altitude: 348 m
G.Speed: 0 km/h
Track: 0 °
Vz: +0.1 m/s
Receiver: LOWG