Some are in the Bigger Pockets book store.

It could be argued that you agreed to store these items indefinitely by the simple fact that they're still there.   

We buy our own equipment for our small retail stores and pay Geoarm $12/month for monitoring.

@Cade Andersen  If you are going to do it yourself, make a list and go to a building supply store to price it out. 

When setting aside money for expenses that aren’t necessarily incurred monthly such as capex, repairs and vacancy - where do you store these funds?

They will also set up a master key / tenant key system for you, so you can have one key that gets you into all of your properties, but each tenant's key only gets them into their unit.A couple of the locks took me a little longer to install in the door than "regular" Kwikset/Schlage/etc locks from the hardware store, but that's because I decided to install a couple of extra screws that the "regular" locks don't usually have. 

Sometimes either the drain valve (on the side near the bottom) or the temperature/pressure relief valve (either on the top, or on the side near the top) starts leaking, and $20 at the hardware store solves that problem.If it doesn't leak, but doesn't get hot: if it's gas, and has a pilot light, sometimes the thermocouple burns out; this is a $20 part at the hardware store. 

But the unit sizes are around 16’x50’ so pretty much whatever you can store in that space! 

They still owe that other person money and are storing their stuff in the house.

Throw a loading dock and being able to convert part of the building to open warehouse could benefit you.