Why do people store things like gun in storage units that 1) are obviously places where people store stuff, and 2) are usually secured only by a padlock?
TF basically has the answer. Especially for people moving from another city or state. I don't know how many of my customers are storing guns without using a safe, but I have seen gun safes go in and out, particularly in the climate controlled units. As far as security...if you know a particular unit has guns in it, that's one thing, but otherwise you're generally talking about several hundred identical doors, so the risk that YOUR particular unit will be hit is lowered. As well, if the facility has fences, gates, lights, and cameras, random burglars are inclined to go to facilities that do not. In my local area we had a guy running around breaking into storage units, but he concentrated on the ones with no security other than the door lock -- he could drive up to their doors right off the street. Broke in, rifled around for small valuables like jewelry, and moved on. Still got caught eventually (turned out he had just got out of prison for doing basically the same thing. Rehab didn't work, I guess).
I have sold contents of a number of units, but not run across any guns yet... Lots of dusty Christmas decorations tho!