I carry 3 mags, 2 on my person (1 in the gun), 1 in the car or [travel]pack. My theory is 1 to defend yourself, 1 to retreat, and the third to get you home.
I can see how a small extra supply of ammo in the car could be useful in the context of getting oneself home, there is usually a few places in most vehicles to stash a small box, it wouldn't be a big deal. Road trips and inclement weather seem reasonable. Seems logical to consider help or police may not be always available after an incident.