Service Engine light comes on. Diagnostic showed no problem. Now smoke fr engine

What is your question?

Replaced alternator. Week later "Service Engine" light came on. Had garage run diagnostics - no problem showed. Only drove on short trips in city and light still on, but no still no problems ... Until I took a 200 mi trip and smelled something burning on my return trip. when I arrived home and turned car off, noticed dark smoke coming from under hood -- not sure where it came from (dark). Just wondering if anyone knows a reason.