Ohhh, good point about the mailboxes. I'd forgotten about those, since for myself, I'd never consider using them as a 'storage house' type option. But I've also seen them repopulate themselves now and then. At first I thought maybe they were just ones I'd missed, but no...they do very occasionally seem to refill. Makes sense in terms of the game, they're getting mail, right? heh.
It's curious that they have the no respawn and no noted associated script...I guess there's some other magical scripting/code the GECK lists doesn't make note of, for those. Perhaps in some odd way they're not considered the same type of container as things like chests and desks. Who knows.
If you check an individual mailbox entry you will see it has a 5% chance of having a magazine in it when you 'spawn' an area by entering it. Since they are marked to not respawn this is presumably for the first time you enter a given location, however it would appear there is a glitch in the code somewhere that checks against that chance
each time you enter that location, subject to the respawn timer. I did not look closely at the trash can entries, I will check one and see if they also have a chance of an item listed.
EDIT: I examined a few trash can entries and saw that they have two leveled lists to draw from that each have a 15% chance to occur, so the aforementioned glitch may be happening with them as well. Now, the vast majority of trash cans are indoors and interiors generally do not respawn, so there may be another mechanic at work here. I did not attempt a comprehensive review of all containers and their lists, since that would take weeks for one person to do and I don't have anything to check such a review against without access to the base code.