nope, you can stick it as much as you want. your savior will be 3M command strips. they are amazing. move them as much as you need.
keep in mind when you are mounting what the field of view of the sensor is. I apologize for the crudeness of the pictures.
so, the red lines ( not perfectly to scale ) are the field of view of the sensor mounted at about 7 feet. you mentioned that you got better results when you mounted it lower.
what might help you is to make a bracket that you can angle down. that way you can mount it high and still get the field of view you want without sacrificing the height you want. the yellow lights are roughly what you would have if you mounted it at that same height but were able to mount it angled down. thus capturing more of what you want but at that height. its not really about the perfect height you are looking for but the angle of the field of view for the situation you are monitoring.
I actually made a bracket that fits this description and has the ability to angle.
check out that thread, maybe its something you can do that might solve your problem, and could be mounted with screws or the 3M command strips.