Thor: Dark World not exactly spoilers, amusing speculation
Now I'm thinking of the security briefings the guards were probably given about Loki in the dungeons.
"If you look in the cell and don't see him, call an alarm to search the area, and DO NOT OPEN THE DOOR TO HIS CELL. He's probably invisible, and waiting for you to open the door so he can escape."
Thor: Dark World not exactly spoilers, amusing speculation
"If you look in the cell and he's dead, wait for him to start decaying. Call in an expert who can tell you if the decomposition of his corpse is accurate. If it seems to be legit, DO NOT OPEN THE DOOR TO HIS CELL. Mark him dead and don't bring any more meals. He'll either drop the illusion out of hunger, or continue to be dead, and either way it's probably safest to just leave him there."
Thor: Dark World not exactly spoilers, amusing speculation
"If you look in the cell and he seems to be lying there dead, or is having some kind of mysterious illness, or being attacked by anything, DO NOT OPEN THE DOOR TO HIS CELL. It's *almost certainly* an illusion, and as soon as the door is open he'll escape."