The only counterpoint I can think of is that she didn’t put the tracking device before, only bc of this event, after Amy got into trouble and no one was able to locate her. She then proceeded to use the tracker to make sure she got home safe, so for now it doesn’t seem malicious or obsessive in nature (although it is definitely without her consent and knowing Bibi, unless they deal with her issues, it could become unhealthy). And Bibi gave her her key too, just in case she needs a safe place to go to (=you don’t have to ask, you’re always welcome), so yeah, it really seems to be out of practicality, prompted by the kidnapping and to keep Amy out of the anguish of worrying her aunt.
On the other hand, pretty interesting of Bibi to do that bc once again, it shows her taking interest in someone and being open to spend time and energy on someone that she isn't consciously enamored/obsessed with. She's often done things for her own gain, so it's nice to see her doing things for someone else's sake (lying/giving her an alibi, driving around town looking for her, using a tracker, giving her a spare key, and previously shown to cook her food that'd be easier for Amy to eat considering her face/jaw injury). Ofc the ethical aspects of these measures are in question, but it's still Bibi lol. But still fun to see her willingly help Amy, even if it doesn't benefit Bibi directly.
Thank you for the updates, it's been an absolute joy to read and I'm completely hooked!