From what I gather the two most common ways to permanently disable a "pesky" CEL is to remove the bulb or cut a wire at the ECU.
If the bulb appears to be in place and if you can apply power to it and it lights (proving it isn't simply burnt out), then check for a cut wire at the ECU.
The ECU should be behind the trim panel in the RH footwell. This video shows how to remove the trim to expose the ECU and the connector where you would look for the cut wire that might be disabling the CEL.
https://www.youtube.com/watch?v=IHtfewneRwE
The wire harness is seen at about the 1:20 mark. Should be easy to see if a wire has been cut.
I'm thinking that the plastic push pins that hold the trim panel might show evidence of having been previously removed as an indicator that somebody has been in there before for the purpose of clipping a wire.