[84LX89GT]

sorry sicy i don't browse as often as i used to, even though it seems like i'm the one making all the posts now and no one else is here. It's good to know there's at least one other person who peruses this message board
Anyway, there are a ton of reasons for a check engine light to come on with out any symptoms especially for an OBD-II car like yours ('96+ mustangs).
First before you bring it in, check and make sure your gas cap is tight - having it loose will cause an evaporative emissions code and send the check engine light.
OBD-II is an emissions based fault sender and the Processor (powertrain control module) will attempt to adapt to make it run right, so even with a fairly decent fault your car will run alright, which masks some of the problems it may have.
My best guesses are that the DPFE sensor could be bad (Delta Pressure Feedback exhaust sensor), there is an evaporative emissions fault, or maybe there is a vacuum leak causing Oxygen sensor or lean codes to be sent.
Those are the most common things i saw working as a technician at a ford dealership, but it's hard to guess without knowing the codes or what the PCM is seeing.
Best of luck, i hope this helped somewhat