If it is on because something is unplugged for example, you could try to reset it and it will come on straight away.
On some cars you can have codes already stored in the ecu without turning on the light, but the light will come on after certain conditions are met long term.
On mine, I don't know if a wire was loose or not, but when my o2 heater wasn't working like it should, the MIL wouldn't even turn on when ignition lights are on, then after awhile would come on and stay on, with a code stored all that time... don't think it's the bulb playing up, maybe it was programmed to do this to say it has detected a problem but is not quite sure yet?
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