You are missing the point, if the entropy is sufficiently low then it is feasible to guess.

Besides, presumably if you want to kill a particular person, you might know a bit about them.

Anyway, with low entropy serial numbers is that potentially it could be feasible to just create a device that runs through all of them in a matter of a couple of minutes or so. For example, you could check google news to get a guestimate of approximately when perhaps a high profile politician had one of these installed. If this is a friend or family member then that step just gets even easier. If part of the serial number is a year/month combo (a common way to do it) and the rest is sequential, then it will be pretty easy to figure out. Are there easier ways? Sure, I imagine so. A hands off wireless approach certainly is appealing though isn't it? Probably worth at least trying before you move on to more hands on techniques.

"it would be a lot faster to do it some other way "

If you are taking the time to plan out a homicide, which is going to be more important: doing it fast. doing it so you don't get caught.