So this is a tiny one. I was in caves with a flight build, sensors, and a signal interpreter. I was moving fast, about 374% or so. I was doing some digging and a cave-in destroyed my signal interpreter. On my screen, I could still see nearby robots that were out of sight represented by their tile as opposed to the "?" mark. I moved about 2 more spaces and could still see their tile. After a third move or so, they became question marks.
What I suspect happened is that since I was moving so fast, the game still showed their tile on the turn that signal interpreter was lost, and only switched to show the "?" after a game turn passed.
Like I said, very minor issue (if I'm in fact right about what I observed, I wasn't in a position to reproduce)