lookout
noun /ˈlʊkaʊt/
  /ˈlʊkaʊt/
Idioms - a place for watching from, especially for danger or an enemy coming towards you- a lookout point/tower
 
- a person who has the responsibility of watching for something, especially danger, etc.- One of the men stood at the door to act as a lookout.
 
Idioms 
be somebody’s lookout 
- (British English, informal) used to say that you do not think somebody’s actions are sensible, but that it is their own problem or responsibility- If he wants to waste his money, that's his lookout.
 
be on the lookout (for somebody/something) | keep a lookout (for somebody/something) 
- (informal) to watch carefully for somebody/something in order to avoid danger, etc. or in order to find something you want- The public should be on the lookout for symptoms of the disease.
- The guards have to keep a sharp lookout for anything suspicious.