Just to complete things - FRA TXL AUH was a C-C connection at Tegel. And yes you need to have your wits about you to spot the staircase leading to airside connections.

The oddness was FRA TXL operated by Alitalia aircraft and crew. An AB-AZ marriage was never going to be a happy passenger experience. We ended up leaving late, then abandoned for 20 minutes at our distant parking spot at Tegel, waiting for stairs, then buses.

So no time for the Tegel C lounge experience