Detection is probably via a reed contact. Take a neodymium magnet and position it in the rear area directly behind or slightly within the radius of the transition from the left side wall to the wall at the back side at the height at which your waste water tank has its magnet. If the app recognizes the magnet, it is the tank, otherwise it is the software or a part in the base station.

Of course, you are responsible for your own actions and I take no responsibility for anything you do with your robot.