I was once suspended under a parachute in Cyprus when I heard an AAC Souix coming my way, I couldn't see it but I heard it slowly getting closer, not a pleasant experience.
Gliders too! Several times I've been drifting about happy as Larry (Larry being a very cheerful chap by the way!) and then heard some spam-can heading generally my way. Could I see them??? Usually, no - which was fine - but I oft thought that, had it not been for the silent flight in my glider, I'd have been none the wiser as to their proximity. And that they'd be clueless as to my presence.
Like, when transiting back in a training spam-can myself in open FIR with a stude beside me and a twin let down out the clag directly in front of us - fortunately, going my way so it disappeared off ahead. Less than a mile further on and I'd have been none the wiser - just wondering what the hell had hit us as we spiralled down to my end (and that of my stude).
Or the time when I was a pax when, suddenly, the sky was full of thermaling gliders! Twenty seconds later and the sky was clear again. Sheesh, and people worry about Covid! "When your time is up...." - never a truer word. Airmanship - zero! Chance - 100%!
PS Medically grounded so now safe - unless from falling debris!
H 'n' H