Any time I am away from my treasure I worry about it too.
A cold winter could change the mind of a kid with a screwdriver thinking it could be a good idea to enter that way.
On the other hand if you were surrounded by other units and a renter decided to unfasten his wall and see what you have ,that is the same threat times three.
If one of the surrounding 3 units were to catch fire, well that 's only twice the threat really, and who wants to set their stuff on fire? Well maybe if it was over insured or a crazed ex thinks it would be a good thing to light it up...how likely is that right? You'd have a better chance of an explosion if the neighboring storage units were used to cook crack....that only happens more than we think it does...so what are the odds?
And here's the longshot. Since most security cameras focus on ground level activity, a smart thief who knows what you have because you didn't bring it in quietly, might bring in two box trucks and park them with the overhead doors facing each other close to the building so the cameras can't see them climbing between them up to the roof where they unscrew the metal roof panels apart and remove the contents of the unit through the roof and into the 2 trucks. Don't worry, Only heard of it once or twice. Okay maybe three times at the most.
Short of putting in your own security system in that would notify both you and the police in the event of a break in , just how is anyone supposed to get a good nights rest when your treasure isn't in your line of sight and out of range?
If I had an issue like this and had 40 extra feet of yard, I would be thinking an overseas container or a pod for temporary use if it would relieve the worry of it all. My neck of the woods are covered with those things and everyone knows to stay clear of another man's container.