Check the gas tank cap for correct seating. could be that you did not re-install correctly it at the last fill up. Otherwise I would not be concerned if the CEL stays off. The CEL on my Wife's 96 Nissan Maxima will come on for a few weeks and then shut off without any help ($$) from me. In the beginning I would take it to the dealer and be charged for a new O2 sensor. Lately I just wait a couple of days/weeks until the computer makes an adjustment, and the light goes off. This Maxima has 125,000 miles and other than too many early 02 sensor replacements it has never given any trouble.
Mark