To make sure that all areas of the building are covered, search the area systematically, using a consistent search pattern, such as bottom-up/top-down or right-wall/left-wall.
Following a pattern is very important in poorly lit areas. If you must leave the building, turn around and reverse your search to get back to your starting point.
For example, for a systematic right-wall room search, use your right hand to follow the wall throughout the building. If it becomes necessary to leave the area before the search is completed, turn around and use your left hand to follow the wall back to your starting point.
A right-wall or left-wall search can work equally well. The important thing is for everyone on the search team to know what pattern will be used.