Check Engine Light Came On After OBD Testing

I have a 98 Breeze (123,000 miles). I took my car to have the annual Vehicle Safety Inspection & OBD Emissions Inspection performed. No problems were found except for a headlight problem. I drove straight home (approx 5 miles). Two hours later I had to run an errand (approx 5 miles). I was at my destination for about 15 minutes. I started car and the check engine light came on. I took the car back to the garage. They assured me nothing they did during the inspection would cause the light to come on. The mechanic checked for a code. First try, no code. Second try, he got a code (I simply can't remember the code). Third try, no code. Fourth try, no link. He reset and told me to drive it and check back if it came on again. That was Friday. On Saturday, the light came back on. Can anyone give me any insight on this problem? Do you think the mechanic is being truthful? Thanks!