How do they detect whether an experiment being done on a smaller model is used to improve a competing frontier model, or just an innocuous hobbyist LLM experiment?

Given how well the cybersecurity safeguards work, they probably don't.

infering the surroundings, like everything else. they will probably look at which company is your email, and if you wrote "better than claude" on the readme.md

this is LLM, it's not like a science or something.