Albert Barnes Commentary Acts 5:23

"saying, The prison-house we found shut in all safety, and the keepers standing at the doors: but when we had opened, we found no man within." — Acts 5:23 (ASV)

Found we shut. It had not been broken open; and there was therefore clear proof that they had been delivered by the interposition of God. Nor could they have been released by the guard, for they were keeping watch, as if unconscious that anything had happened, and the officers had the only means of entering the prison.