Years ago I had problems with thrives stealing fuel cans from my business. At the suggestion of a employee I prepared a "special" 5 gal. fuel can that contained roughly equal parts: contaminated used brake fluid, hydraulic fluid, diesel fuel, water, low-grade gasoline, 1 cup of fine metal shavings and finally 1gal. of household bleach. I put the container in a conspicuous place close to the security fence and marked it with a Sharpie pen "Premium gas only!". A few days later the can had grown legs and was almost certainly added to the thieves car. I kept hoping I would see someone close by with a tow truck hooked to their vehicle but no dice. I still got the satisfaction that if they did pour it in, their vehicle soon suffered catastrophic failure. At least the thievery stopped immediately.
Since your plant appears to be happy and thriving, it wouldn't be a stretch that they may return. A special concoction labeled "super duper fertilizer" or other could easily be prepared and left casually outside yes? The biggest danger would be if the thief actually poured it in YOUR plant I suppose, but, the idea of some asshole unwittingly pouring a toxic potion on his grow has its appeal.