Thank you for your feedback and possible solutions.
I was able to get something working using the “last scan” field and searching for “unknown” in the string.
This is not an ideal solution but does what we need.
When my colleague scans a barcode, he will either hear a “positive” sound or a “negative” sound.
If he doesn’t hear any sound at all, he should then turn his attention to the screen because there’s likely an error on screen.
I will look into TamperMonkey and see if it’s worth pushing this idea further.