I know we weren't discussing this, but now this counts as a mystery for me and I need to know the truth. Sorry about that. ^^"
After Bill left the hotel, the group split up for some reason. At some point, Howard ended up in his own room behind the reception.
Lucy went in Howard's open door, then forced Howard to commit suicide. After Howard was dead, Lucy took the masterkey from him, she went out the open door and headed to the basement where she threw the key under the door.
At this point, Charlie may have noticed the dead body in Howard's room, because he fled in a random hotel room in panic, locking himself inside.
When Lucy came back, she locked Howard's room from the inside with a key that is not the masterkey. Lucy left the key in the room and escaped the room by using a secret passage.
Possibly Charlie only took his turn after Howard, but even if not, that's not yet related to my blue truths.