I don't have an AirTag, or any Apple products, but I've read on posts here that a thief's iPhone (if they have one) will detect the AirTag. I don't know if they can manage or disable the thing, but they would certainly start searching for it immediately!
Does one need an iPhone? Is there an Android AirTag app?
Does one need to remember to mute AirTag alerts each time leaving the garage?
AirTags are for Apple products only.
There are Android based trackers as well very similar in size and function. Among those released or on task TO BE released include products by Chipolo, Pebblebee, Tile, Samsung, Kzo and more. All for sale on Amazon.
An AirTag placed in a car will "log in" so to speak, by any passing iPhone, without that passing iPhone being aware of it.
However, if the AirTag is "following" one specific phone that does not belong to the owner, yes, that phone will be advised that there is an AirTag "following" them. So, if a thief steals your car with an AirTag in it, as soon as his phone is near the AirTag, that location is noted, you are advised as to the location...and if the thief has an iPhone, they will eventually be notified that they are being followed. But, by that time you already know your car was stolen and have a general idea of where it may be.
Every time my Pagoda leaves my garage--it's "home base"--I'm advised of that. I'm kept advised of its location unless it's with me. So, if I'm driving it, no notifications. If I park it somewhere and walk away, I'm told that it's left behind.
AirTags were not designed to be perfect, they were not designed to replace GPS trackers, but for $29 they do a darn good job of telling me where luggage is, for example. They won't prevent a thief from taking your car (apparently a locked car in a locked garage won't either) but they will give you an idea of where the car may be located.