i find this works on lenovo laptops. Lenovo laptops like to sleep rather than fully shut down, since no matter your settings, closing/opening the screen puts it to sleep/ wake up. So the laptop is always powered on (you'll see the logo light flashing).

But at best such detection is spotty. What most likely happens is that thief is scouting area, and you park your car put your stuff in boot, thief sees what you put in boot and makes a move. So if you want to strategise, organise your stuff first, and do not open your boot when you get to location, only when you leave said location.

Also make sure your hardware are powered down, not in sleep mode.

I had tools stolen before, it was a sling bag but totally did not resemble laptop bag. I just had tools in it and workers at the service center for my car stole it because they went through it during cleaning despite my protests and stole it at that point. Since this is not official place well i couldnt get it bag nor any reimbursement for it. I found my cars locks broken as well.

Thieves are somewhat dumb in a sense. They don't check if its a laptop, if they see a bag in your car they act, so if they see you open your boot and see what you have in there/put in there when you get out, thats how they know. Theres no special detector, but also just make sure your equipment is powered off as detectors only work for carthieves who copy your car's alarm. Some cars are vulnerable to alarm command copying via wireless so these thieves have said special equipment and may just steal stuff than your car (depends how profitable it is for them to steal your car vs your stufF), and can gain access via the wireless alarm if your car does not use a kerberos style alarm in which a chip sends challenges wirelessly in which the response is never the same everytime you press the button.