Mexico without a visa?

You are from the USA, right? You don't need a Visa if your staying a short time at the border towns. You do need a passport to get back in the USA. There IS a difference. Google it. And to answer your question, I don't care what they do, I take my passport....not taking a chance in not having one. Doubt it would be a Mexican jail....those are US border agents making sure illegals aren't coming to the US.

Not sure you need a 'visa'. The rules are documented at Mexico Entry Rules (monterrey)

These may or may not be up to date; just ones I found searching. If in doubt, call the Mexican consulate and ask :)

Probably a lot more harsher than for a Mexican being caught in the U.S. without a visa! :R

You know those stories about the Mexican jails?
Don't drop the soap

Large Fine!