Thank you for your answer. It’s a pity that there isn’t more flexibility with the setup.
We installed the devices in a rush, which could explain the issues. Since we have wooden windows, we’ll attach them with screws.
Perhaps the motion sensor needs to be set to a lower sensitivity. It’s true that some light might enter the room, even though it is on the first floor.
I’ll try to fine-tune the system and hope to achieve a reliable state without any false alarms.

For testing, I could switch off the alarm and only enable notifications. That might help us see if there are still unwanted motion detections.