Originally Posted by
TravellingSalesman
Wouldn't the exit stamp here refer to the one you get when leaving Schengen? Presumably for CBP it is only important when you left the Schengen zone, not when you entered or left Mexico.
Schengen definitely has exit controls. If (unlikely) the border guard does not stamp your passport at the exit control, you can ask them to do so. This should provide you with the required proof of having left Schengen more than 14 days ago when you arrive at the US border.
EU Citizens usually go through the automated border control, so no stamp. Permanent residents normally go to an agent and depending on the agent's willingness, they will stamp it or not. I had to make a strong case at CDG once to get a stamp since the agent and his supervisor both concluded that I didn't need one.