John Gill 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)

Saying, the prison truly found we shut with all safety ,
&c.] Locked, barred and bolted;

and the keepers standing without before the doors ;
both inner and outer in their proper places, diligently discharging their office:

but when we had opened we found no man within ;
that is, none of the apostles, for there might be other prisoners in it, who were not released by this means.