So I was digging around a dungeon, and when I got to the door of it, I noticed it was open.
"Someone must have done a really terrible job at raiding this" I thought.
I went in and suddenly the protector disappeared. And then another. And then another. And then the chest was raided and a hole formed at the end of the tunnel.
I asked the most recent person whom had raided a raided a dungeon based on the chat, if I was just in a dungeon with him, and then notified him that he was invisible on my screen.
He did not answer, but I noticed he very quickly and suspiciously went on to raid another dungeon and repair a teleporter within the timespan of about 3 minutes.
I don't know if this is a bug or a cheat. I remember the persons name, just in case it's a cheat., but I do remember invisibility being a bug in the past.