Don't know if you have been in similar situation but for me it happened long time ago. Not talking generally about iphones but about gadgets. I had problems with my laptop and smartphone, went to service center and baaam no issues, they couldnt replicate issue, everything worked fine. Later came back to home and same issue appeared again... ;D
That seems to happen a lot. It seems that there is some kind of magic Wi-Fi dust in the air at the Apple store.