This is true, I was on patrol on venus where I saw a guy standing by a half risen spire with no praetorians spawning or other vex spawning. I watched him stand there whilst the spire built on its own, he then ran inside the vault on his own; I later got the message at the bottom of the screen (when someone gets a legendary or higher) that he had got some shards, presumably from the chest. There was no fireteam anywhere, the only people in the game was me and him. Edit: I did the "interact" command, which allows you to see what activities they're doing, his said they were doing a patrol.