My parents are away and my sister, her friend and two guys are here. They went to the other end of the house....
How can I find out?
Easy, put on all black clothes, a goalie mask, and carry a butcher knife. Splatter some fake blood on yourself and the knife.
Go in each room in the house and start looking under beds, tables, chairs, etc. If you are asked what you are doing say you are completing "unfinished business" and continue on.
No one will stop you and you should disturb anyone in the mood enough that all activity will stop.