10When they had passed the first and the second guard, they came to the iron gate leading into the city. It opened for them of its own accord, and they went out and went along one street, and immediately the angel left him.
The angel leaves Peter after they have walked far enough from the prison so that there is no immediate danger from any guards. Since it is night, there is unlikely to be anyone on the street to see them and Peter is now free to go where he wishes.
10 When they had passed the first and the second guard, they came to the iron gate leading into the city. It opened for them of its own accord, and they went out and went along one street, and immediately the angel left him.