Other than emission-related component failure, the easy things to look for are:
1. Have you recently fueled up with 87-grade octane?
2. Did you close the gas cap firmly (three twists followed by that ratchety "click")?
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car