I appreciate the replies Kakugori and Blarghity. I thought it might be vermin too. But I have zero evidence of any vermin. The storage unit is free of droppings, and there's not any evidence of chewing in any of my boxes. The fog machines are stored in the black and yellow tough boxes pictured in the attached. The other pictures show examples of the fog juice damage:There is another option besides combustion. You are putting this stuff in offsite storage, I assume some sort of rental unit. Maybe the place has vermin,which would be drawn to the fog juice the same way they'd be drawn to the highly toxic ethylene glycol still found in many car antifreeze brands. Once the fluid leaks, and is exposed to filthy vermin, it would decompose naturally and produce the staining you're seeing on the reaper.
Incidentally,I blame left over fog juice for a rodent infestation in my garage about 8 years ago. They go after partially used containers, but I've never seen them damage any full and still sealed containers.
This is an example of the fabric "burn" marks I described.
This picture shows how the fog juice container sits in the metal fog machine housing. There's no way for vermin to get at the bottom of the container because it's all metal in the back of the machine. And there is less than 1/2 an inch clearance around the plastic fog juice bottle.
This is a picture of the burn hole (now patched with white liquid nails) in the fog juice container. You can't really tell how it looked originally, but imagine a hot poker going straight through the bottom corner. The plastic had clearly melted and burned in this corner.
This last picture shows the black and yellow storage bins that have held my foggers these past two years. The white substance on the top black box is chalk that got on the box while stacking the boxes.