I took Zak into the spare room and waited until he was warm, then I put him on the curtains and turned out the light. After about 10 minutes he was looking around the rooms. I stood close and I could hear a faint buzz: he was pinging.
Then he flew.
I let him zoom and swoop for half an hour, then turned on the light and caught him.
I'll have to check the 4 day weather forcast to make sure there's no cold fronts coming. If it looks good, tommorow night he goes.