Ever consider the so called "poisoning the well" method?
We had kids stealing fuel out of local farm equipment a few years ago.
One of the more clever local farmers left several fuel cans sitting by his farm equipment one night. Two were full and the others were empty.
The intention was to look like he had been fueling up but forgot the cans and had left them out.
They were the plastic jug type cans so this worked very well by the way. As it was explained to me he filled each one with about 4.5 gallons of E85 fuel and .5 gallons of muratic acid. (hydrochloric acid).
Hydrochloric acid reacts violently with the aluminum in the fuel system and the aluminum pistons and other engine components and the high ethonal content of E85 keeps the acid in suspension so it will stay mixed with the fuel for some time without the water and acid part settling out.
The next morning the cans were empty and one pickup and two cars were sitting along the road a few miles away. They each had puddles of fuel under them too from the fuel tanks corroding over night.
A little asking around later that week revealed that all three had the engines go out that night as well, and all within about a mile of each other.
The three owners were high school kids!
If you do the E85 and acid mixed with about half diesel I hear that it can destroy diesel engines too!
If you set it up right, I bet you will permanently stop the fuel theft and find out exactly who it is thats taking it!