My 99 Sonata lit a P0455 on Sunday night, cleared it, no sign of any reoccurance.
Explain to your other half that since it went out and did not return, it was likely a sensor that threw a wonky signal, once, and that it shouldn`t return. If you clear it and the light comes back immediately, or very shortly (within say 24 hours) then is the time to take it to the mechanic, because it is indicating a problem.
Since check engine lights are SO precise (your car is scanned a few times each second, at least) if it gets ONE signal out of spec, it`ll trigger that light. Cold night, EGR is slow to move? CEL. Hot night, humid in the vacuum line? CEL. There`s at least $120 in a shop`s bank account.
If you clear it and it stays away, write it off as a one-timer. Keep a record, so if you happen to notice a trend (it gets below 50, you get X code everytime) you can act accordingly. If you know car basics, these new on-board computers aren`t so hard to understand.
Hope I`ve helped (with the car and the better half).
Ryan
(PS - if the light has stayed out and you`re confident, cancel that appt.)
