Assuming this took place on an Airbus, it would still be interesting to know what type of Airbus.
If the additional input is very short, there will be an optical indication on the glareshield in front of the pilots which, unfortunately, one is likely to miss in high workload situation. Only after that will an acoustic warning be given. I'm not really certain about the amount of time, I'd have to look it up. But I'd guess it to be a little less than 1 sec.
That's for the technical side.
On the other side, I wouldn't take such a comment too seriously. The Copilot not noticing or not complaining about it are two different things...