stand guard
To watch over someone or something; to act as a guard or a lookout for someone or something. He stood guard outside the bathroom to make sure no one walked in on Jeremy getting sick. The police are standing guard around the prison, lest the prisoner should attempt escape a second time.
stand guard
Watch over, act as a lookout, as in We'll climb the tree and get the apples if Josh will stand guard, or There's a parking space; stand guard while I make a U-turn and get to it. This term alludes to the military defense of posting guards to watch for the enemy.
mount/stand/keep ˈguard (over somebody/something)
act as a guard: Two soldiers stood guard over the captured weapons. stand guard
1. To keep watch.
2. To act as a sentinel.