Perennial topic this.
Few weeks ago there were a lot of gliders up and I (in a 152) saw one very late. (I don't know when s/he saw me; s/he had better visibility and was better able to predict that I'd be continuing straight and level than I was to predict their course, so I'm hoping there wasn't really a danger of collision.)
Might have helped if they'd had any radio - the glider field I was passing only advertised a mobile phone number with the helpful comment that "we probably won't bother to answer this number if we're too busy flying" (so I didn't bother calling it before taking off).
For example, if someone had been there to tell me "we've got five gliders up in the local area" I'd have known that when I'd spotted four of them there was still one to go.