Today I saw a sign above the dropbox: "please park your returned trucks <in the lot>"
So I think what's happening is: U-Haul keeps the vans in the strip mall parking lot. (Do they have to pay the mall for that? Who knows?)
Possibly some of the mall tenants rent it occasionally, and this lets them get the van right at their place of business, instead of having to go somewhere and pick it up.
Maybe the keys are always left in the van! I could test that, but I don't want to get in trouble for car theft. Maybe U-Haul doesn't have to worry about someone stealing it, since they could have a locator device in it.
It would be some effort to figure out why they're doing it, so I haven't done so.
Wait! I've got it: U-Haul is just parking their rental vans there instead of their own lot. So the vans are actually NOT rented at the moment?
Stay tuned.