I got that something else once. And that something else was NOTHING.
Heard the sound of my front door opening. Looked out the window, didn't see anything. Went to investigate. Nothing. Then I heard another door down the hall open. Went to investigate, still nothing. Was already getting freaked out, headed back to the main room just to see the door to the garage open on its own, followed by the door to the main room. Something invisible was running around my base. Dunno what it was but it eventually ran into me and crashed the game. Was the most terrifying experience I've had in this game in a long time.