Acts 12:9

And he went out, and followed him; and wist not that it was true which was done by the angel; but thought he saw a vision.

And he went out and followed him. He didn't know that what was being done by the angel was real, but thought he saw a vision.

And he went out, and followed; and he knew not that it was true which was done by the angel, but thought he saw a vision.

and having gone forth, he was following him, and he knew not that it is true that which is done through the messenger, and was thinking he saw a vision,

So Peter followed him out, but he was unaware that what the angel was doing was real. He thought he was only seeing a vision.