This is an odd question, perhaps; but first some background. I have an allotment garden (in UK) that I enjoy putting a fair bit of energy into, growing unusual vegetables and fruits. However, my enjoyment is marred by people regularly stealing my produce, so I'm thinking about how to discourage the thieves, and one idea I've had is to spray the plants with something harmless that leaves some sort of visible residue, and the put up a notice warning that the plants have been sprayed and possibly a recommendation that they contact the emergency department immediately if they feel sick, tired or have stomach symptoms.
But of course, there may be reasons why this is not a good idea; and if it is, which substance should I use - soap, chalk powder, ...? It should be something that is harmless and easily washed off, of course. What do people here think?
Alternatively I could put up wild-life cameras, well-camouflaged and with SIM-cards, so they live-stream pictures, but that feels a bit extreme (as well as expensive).