20 plus years ago storage unit with key code knew full well if you entered and never left. So drive in and stay, they know it. May not do much about it, or they might. I rented a big one for a spare car I could not keep in the parent's driveway and a place to work on said car. They knew I was there for a lot of hours, kinda became a hangout with some friends I had at the time. But they also knew I left. Came up in a discussion when I was paying rent one day cause someone was living in unit and they knew it and I knew it and we both sorta wanted to see if the other knew it. And they knew I worked on my vehicle there, I left door up when working on it and they stopped and chatted now and then.
It had a power outlet and I asked because using it much cause I am that sort. They had a few folks who would try to pull too much power and pop the circuit. Amazingly enough, some folks never got the circuit reset and since it was not in the rental agreement there was not much that person could do. In each and every instance, if it popped rarely they reset. You make a habit out of abusing it and don't learn from it popping then they let time teach you a lesson.
Another unit had lights on a timer. Kinda cool but loud sucker.
I have had stuff stolen out of a storage unit, they too advantage of a corner of the fence and got the closest units to where they came over. Only took stuff they could easily toss over fence. I stopped in cause I saw the police there. I never got a call about it and was on way to work when I saw police vehicle and swung in just to see what was up.
In looking at design of most units, anyone who rents one unit can probably dismantle a panel or wall and get into adjoining units. Your call on how secure you consider that.
Not sure what else I think on this, don't keep everything in one basket but do keep in mind a basket you don't check on may not be there when you need it. Youtube has more than one fella going to check on his "y2k stash" or whatever shtf stash and it is no longer there. Or they just can't find it. As far as putting stuff in the ground, something like a 55 gallon drum is going to want to raise up out of the ground depending on ground water and what not. Drainage and all that can be important. Does depend on your ground.