If you allow this to go as far as it can, you end up not able to use anything. Every time you buy something with a USB connector on it, it might have been compromised. Every time you are given something, that might be compromised too. The computer you bought might have malware preinstalled. The parts you were going to use to build yourself a computer because you can't trust the manufacturers might have malware on them.
In the case of the conference, I think it's fair to assume that the drive is probably safe. Don't just assume that it is--test it first--but it is not the high-risk situation like when drives are found unattended. If you always use "what is possible" as your question for trust, you will end up at a dead end. Instead, ask "what is feasible" and "what is likely", and take whatever precautions are available for those infeasible and unlikely things that nonetheless are possible.