After a new start from scratch with remote stands, I recognised that I got one stuck, but invisible passenger at my airport after I all commercial flights for the day went of. As I ran multiple stands before I activated the remote stand, I am pretty sure that the remote stand causes the issue.
The passenger is not findable at the bus stops as well as somewhere at the apron or ramps.
PS: I know, I read about that thing some days earlier, but I am not able to find it yet (like my passenger).
PPS: All ramps used are small ramps.
EDIT:
Side note, after reloading the save the passenger number got back to 0.