Saying, The prison truly found we shut with all safety, and the keepers standing without before the doors: but when we had opened, we found no man within. (Acts 5:23)
If the angel had unlocked the doors, he locked them after freeing the apostles. The guards at the gates seem not to have been aware that the prisoners had escaped. Compare release of Peter (ch. 12:6-10), but contrast the furor and drama of Paul and Silas’ experience in Philippi (ch. 16:25-30).