Speaking in general (not specifically Subaru) about CEL problems, if the
light is not coming on and the car runs OK, you can probably conclude
that whatever was bothering the commuter cleared up on its own.  The
light comes on when conditions meet criteria that the computer is
programmed to view as a problem.  If the problem conditions go away, the
light stays OFF, but depending on the nature of the fault, the computer
may retain fault codes in memory -- at least for a period of time or
number of ignition ON/OFF cycles.  If it is convenient, and especially
if you can find someone like Autozone to do it for free, get the system
checked for stored codes and post the results here for comment.

As a case in point of my previous suggestion to check for vacuum leaks,
on my '96 Outback it is all too easy when changing the air filter to
dislodge a hose at the back of the housing.  Air leaks cause the engine
to idle faster as the computer just puts in more fuel to compensate, but
a vacuum leak may affect some cylinders more than others and thus cause
the engine to run poorly.  Thus, in situations such as yours, it never
hurts to look under the hood for loose or cracked hoses.

Ed P