[DMX84]

It’s not an “octane sensor”. It uses the knock sensor to detect knocking & its not necessarily audible. If you do hear knock than you have a much more serious condition.
Here is some good information on this subject.
The "octane value" is a term coined by Todd Day to refer to the memory address that is used as a longterm timing trim. If the ECU hears a moderate amount of knock for a long time, or alot of knock for a shorter amount of time, it will keep reducing the value in this memory address. Normally set at 255 (FULL), it will keep getting reduced as long as it hears knock. This could be due to just getting a bad tank of gas, or perhaps running more boost than you are tuned for. But, even if you fix the problem, by filling back up with good premium fuel, or turning down the boost, the "octane value" can take several hundred miles to work its way back up to "FULL" value again.
Dean