We found that the PLC code actually contained logic that would lock up the train with bogus error codes after some date, or if the train wasn't running for a given time. One version of the controller actually contained GPS coordinates to contain the behaviour to third party workshops.
It was also possible to unlock the trains by pressing a key combination in the cabin controls. None of this was documented.
2/4
The key unlock was deleted in newer PLC software versions, but the lock logic remained.
After a certain update by NEWAG, the cabin controls would also display scary messages about copyright violations if the HMI detected a subset of conditions that should've engaged the lock but the train was still operational.
The trains also had a GSM telemetry unit that was broadcasting lock conditions, and in some cases appeared to be able to lock the train remotely.
3/4