Sorry if I steal the thread here, but I had a somewhat similar thing happen on my home course.
3 friends and I were playing at Dolese and we get to the 11th hole. We hadn't seen anyone in front of us(at hole 8 we can see the next 3 holes, and no one was there). We get to hole 11, starts out in the open and the basket is in the woods. We see someone walking around down there so we wait for a second, and then he waves for us to throw. I pulled out my FLX pred and threw a scooby. It looks like a good shot, probably within 20' of the basket, one of my friends said he saw it land and it looked like I was parked. Then the guy starts walking around by where we saw my disc land which worried me, so I ran up ahead of my friends to check the situation out and make sure nothing happened. I get up there and can't see my disc. The guy said he saw it(asked if I threw a blue one, so he knew the color, obviously saw the disc), claimed it hit a tree and shot way off to the right(which isn't possible with a scooby, it was coming straight down so it could not have flown off that way). I look around more and wave my friends to go ahead and throw so I have some backup. He keeps telling me my disc flew way off where it couldn't have gone and I look for it there just in case.
Back on the tee my friends threw, one of whom thew a blue Xcal that he had bought that day. After their shots landed I went and looked at them, and saw my buddy's XCal sitting in a bush, having seen it I walked off figuring he would pick it up. We all continue to look for my disc, which should be in the open near the basket but is nowhere to be seen. I ask the guy again if he knew where it was and he said he never saw it(contrary to what he had said earlier). At this point I'm getting very suspicious. I finally give up after 10-15 minutes of looking and as we go to grab my buddy's Xcal it's vanished from where I last saw it. I knew exactly where I had seen it, but it was no longer there, and none of my friends picked it up.
At this point the guy who we suspected of thievery and his friend went to the next hole. We went to the next hole and asked them if they had the discs, or knew where they were. They both said they hadn't, and showed us the discs they had(they had no bag, so we couldn't check that). We ask a few more times and they start getting very defensive. They were both wearing hoodies(in 90 degree weather) so one of my friends asks them to lift up their shirts to prove they didn't take the discs. they refused at first but eventually showed us and they didn't appear to have any discs on them. My friend accuses them again and at this point they get angry and try and start a fight. After much yelling a group of regulars at the course walk up and chase the guys away. After talking the regulars they said the saw the guys earlier in their round, and they attempted to steal one of their discs in the fairway(which reinforces our idea that they took our discs).
After all of this my friend and I are both down a disc, and have no real proof that the guys took them other than situational evidence. Fast forward a week and I show up with the same friends at the course and see one of the two guys we suspect stole our discs. One of my friends(who lost the XCal) confronts him and asks what discs he's throwing, he shows us all but one disc, my friend asks to see it and it's his blue XCal. The guy, with his girlfriend, immediately walk off the course(with my friends Xcal still in hand) and leave.
It was reassuring to know that they actually did steal our discs(and they didn't just magically vanish) but the situation is just so shitty. The guy was obviously just waiting there for us to throw so he could take our discs, and they covered it up the whole time. We still don't know what the did with the discs when the took them, as they didn't have the discs on them. We imagine that they put them in the woods somewhere and came back for them later. Thankfully these guys haven't been seen around the park since, and hopefully we won't see them there again. I just hope that we're the only people they've done this to(though it's doubtful). I still miss my flx pred(best scooby disc ever).