Some wild guesses:
- ramp check by plain-clothes German CAA staff (checking flight paperwork, a/c documents, loadsheet etc)
- Some sort of lease / aircraft ownership problem - there are people who professionally "steal" (back) aircraft which are behind on lease payments (unlikely with Jet2 I guess?)
- Coppers looking for someone on the plane
- Coppers or some other gov't agency giving the crew details of some dangerous / sensitive person / cargo on board
