I spend a lot of my time looking for other gliders. They're easiest to see in a turn. Coming straight at you or away, they can be next to invisible, but if they're turning, the aspect is changing and they come into and out of view.
In this case the glider was climbing with several others -- when a glider finds lift, others join in.
It might be a good idea to put glider symbols, maybe even the frequency, on the low altitude IFR charts where there's mountain wave lift.