Probably if the old bandit had gone, he would have left a rear guard.

Wolf hoped to God the police had not left a guard.

But they left a guard on duty not too far away as the rest tramped off.

You better leave a guard on your boat, Tony, people say.

"I think we should also leave a guard on this place, in case some of them should return."

Also they can leave a guard there and rove at will, to return upon signal.

What if they wouldn't let him leave, set a guard on him?

But, lest my plan should fail, I shall leave a guard to watch you.

Suppose we go to the island, that is, some of us, leaving a guard on board here.

They had left a guard, then, or more properly a spy.