Malfunction light and a 05' Pacifica?

The malfunction light came on this 05 Pacifica about a week ago. Just on not blinking and no differance in the cars performance. Toke it to a shop and got a diagnostic done which showed nothing was wrong and it wasnt giving off any codes or warnings ect (I am no mechanic so please excuse) after he toke it off then started it the light went off. This was 3 days ago and now it has come back on. What could be wrong? Is it serious?