Probably more than twice, but the two that come to mind are from the same course about ten years ago.
1) Three kids were in the water fishing out discs but otherwise no one was waiting on Hole 1, so I took four or five shots. It is a short hole, does not need more than a putter or mid to ace run, but there is a creek that runs through the middle and a few tall trees that can swat down your drive. Still, a fun opening ace run hole. After taking my four or five shots, I walked up and found all but one. A guy parked close to the hole and who could see everything from his car, yell out at me that the three kids had gotten out of the water to watch me throw, and in the time I leaned over to pick up my bag, had grabbed one of my discs and thrown it in the water where they would find it later.
The guy in the car had no reason to lie to me, but the kids denied it emphatically. Playing solo, it takes me about 30 to 40 minutes to play a round. I told them that when I returned, and if they were still there, I was going to search through everything they had, and if I see my disc, I am calling the police. I was not going to actually follow through with that threat, but they were gone when I came back. It was a near-perfect FLX Buzzz. I never could find a replacement, so I gave up on the mold.
2) That winter, Hole 8 is a long, winding Par 5, but the first drive is slightly downhill and slowly curves to the left before doglegging back up the right. The creek is on the left side the whole way, so it is a tough shot to land something where you want it. The park is packed during the summer, but it is much quieter during the winter, giving you opportunities to throw a few extra if you want to test a line.
I took two shots, as did the person tagging along with me. We found his two right away, but I have a bigger arm and was able to get both drives down near where it doglegs back up to the right. I found my first one right away, played from it, and nearly walked away from my second practice drive.
A young teenager, known to loiter around the course, and occasionally play, but mostly a suspected disc thief willing to trade discs for weed, for whatever reason was hanging around the dogleg (it is a pinch point near Hole 9 and 12's tee, so it not uncommon to see other people there). He is the only other person on the course besides us, and he did not look like he was doing much. He placed his backpack down (this was before disc golf had backpack style bags) in the middle of a fairway and started roaming around like he was looking for a disc. I went back looking for my disc. Eventually, I yelled at him that I was moving his bag because I believed that was where my disc landed. Sure enough, my disc was directly under his bag. It was not buried to begin with, so he purposely put his back on top of it. He apologized, saying that he thought it was his disc - right, on the same day as I was there, he was also throwing (which we never saw him throw at all) a pink FLX Surge SS with my PDGA number sharpied largely on the flight plate. Why would he want to steal it anyway considering how I had marked it?
Side note to the second incident - five months later, a friend and I were playing a really late-round there. We occasionally play night rounds here, but tonight was not going to be one of those rounds, so we were rushing to finish up. That same kid was wandering through the course, saw us, saw me, and then wandered off the course. As much as we said we wanted to be finished before it got dark, we talked ourselves back into playing the back loop one more time. On Hole 9, we found his backpack sitting underneath the basket. He was gone, and we waited to be sure. I do not partake, but my friend recognized the stench of heavy weed usage.
I do not care if you judge me or not, but we ransacked the bag. No drugs but about ten discs. The ones that had numbers, we called or texted, maybe five of them, and each one said that it was the first time they had been contacted after losing that disc. We were able to return those and left the others in the backpack.
I do not play leagues there often, but of the few times that I did, I saw him once. I am not sure he recognized me, but I recognized him. He never said anything to me, and I got mixed reviews about him from those I played with. It was ten years ago, so I am sure he has gotten on with his life - hopefully for the better.