Honestly, this is pretty hard to solve without some surveillance. I can think of a few options to try to ameliorate it.
Using psychology: Post a statement appealing to people’s self-image. “Please be considerate. People have spent time and money on getting their lunch ready. Some of us have medical conditions which require a specific diet, and having our lunch stolen is much more of a burden than it might appear. Think about how you would feel if people took your property from your desk. Thank you for your consideration.” Or some such version.
Trying to catch them in the act by going to the kitchen at unpredictable times.
Bringing something that you don’t have to keep in the kitchen. It would work, but then you’d have to accommodate the thief(s).
This is more out there, and I have never tried it. Maybe it would be possible to use some safe food colorant in a “trap” lunch. Whomever has a colorful tongue afterwards is probably the culprit!
