It might work, but the issues probably aren’t the ones that you’re thinking about.
There’s no problem with it being out of range of the hub, and it’s not going to send measurably more traffic or use up the batteries any faster. This isn’t that dissimilar from the SmartThings branded presence sensor.
(edited to add As @ero4444 points out below, One issue to be aware of is that at the moment that the motion sensor loses contact with the network, it will probably be reporting motion detected since presumably you are in the car. That means the status for the remainder of the day until you return home is always “motion detected.” So you need to think about how to design your notification rules to address this issue. But you won’t have reduced battery life because the sensor goes out of range.)
That said, it’s very likely that the extreme temperature variations inside a parked car are going to significantly damage the motion sensor overtime, as well as giving you a lot of false positives. Most of the sensors just aren’t intended for environments that get that hot. But you can check the specifications on any specific device you’re considering.
There’s also going to be an issue with getting signal out of the car because, well, it’s a big metal box. That’s when you can just test. Problem is that the usual solution to getting signal that is to put it near the glass windows and that’s exactly the area where you have the most temperature variation.
So you can try it if you want, but my guess is you’ll get a lot of false positives and you may cook the device.
it works. I put motion sensors in cars for the same reason, douchebags plague many neighborhoods around me. It seems systematic and periodic. They got me 3-4 years ago, entered through unlocked passenger door and pushed it closed without latching, to make the least noise. They got a multitool and junk cables but the rest must have looked like trash in the brief time they took. Always lock your doors - duh - good reminder. Now the wife nags every … day. Thanks - too late.
So since then I put ST motion sensor gen2 in passenger door. It mostly works, never lost contact within the 60 feet that I park. It notifies motion if the door moves, or anything moves in view. You could alarm the back separately, or cover everywhere with different, more-obvious placement.of a single sensor.
when I get home in winter, the car interior cools off from 70F, and the sensor sends me 1-2 spurious motion alert at some point in cooldown, at appx 43F last night. In the summer after engine-off, the interior gets to 100F+ and I think in that “cooldown” it also can report a spurious motion, but less often than winter.
when I leave home, usually the last report from the device is “MOTION” so that sensor indicates (old) motion on smartthings app until I get home so it can update again.
in my case the wife messes with the car, I disable alerts for a period around when the front door opens, or another monitored car arrives. That reduces junk alerts.
battery has lasted more than a year. The hub doesn’t care but that it up to how you zone it.
We have the motion flood lights and even cameras on the driveway. 2nd break in didn’t even flinch when they kicked on, just sat in my car going through the center console, overhead dome lighting everything up as well.
Not looking to have this trigger a full-blown alarm condition, just kick on some lights in the house or sound a chime to let us know that something is happening “right now”. Likely only after hours as they seem to hit from 2-4am.