Upvote:2
It is very unlikely that you actually went through a proper immigration checkpoint.
On my last trip out of Frankfurt in September, I had ample time between checking in for my flight to Hong Kong and the planeβs departure so I spent my time exploring how far I could move around airside. The non-Schengen parts of the airport follow a decentralised security concept, i.e. each security checkpoint serves only a couple of gates. After exit immigration but before going through security, I was able to walk all the way from terminal 2 to the centre of terminal 1 where the security checkpoint for the B gates was. I took the SkyTrain back to terminal 2. As the SkyTrain is separated into landside and non-Schengen airside, it should be possible to ride it to reach the non-Schengen A/Z gates too, all without ever passing immigration.
Aside from the security check (the one where your hand luggage is X-rayed), the only other possibilities I can think of are: