The DM should know the monsters and people in the dungeon, if they are aware of the players and the actions they would take.
If they cleared the room but there were other guys in the other room who heard battle noise, cryes, etc they should prepare and go kill the players, direct approach or with stealth.
If the people or monsters killed in the room should meet another person or or monster somewhere this one should be suspicious that something is happening and take appropiate actions.
If no one noticed players should rest without external interruptions unless DM knows someone could pass and notice them, for example a guard patrolling or a random encounters if the place sees a lot of movement, with a random roll or without it if the DM wants to punish lazy players.