I have a belt that you can slip a phone/ID into. So that's where my passport goes. There will be plenty of pouches/options at the expo.

The risk of not carrying it is a rather unpleasant discussion/dentention by border security. It's not worth that. Don't mess around, carry your passport/border ID just in case.

That said, the only real risk of being stopped at the border is if your bib isn't visible on your front. This isn't the race to bandit -- believe it or not, people do it.