Which ecu?

Ive not sat there and decompiled them so I am being speculative, so dont take my word as gospel, but I would imagine that most modern ecu's will have a profile of expected background noise and look for variations from it, thats certainly how I would program it if I was writing the ECU software.
This is also what I would look to achieve on something like SM4 where you can map allowed values against rpm (or rpm and boost for that matter) and only turn the light on if those are exceeded