FAQ

It's a cue to the police detective to ask a specific question; you will notice that each time he asks a variation on "Did you and so-and-so arrive together?"

A second theory is that it could be a way for him to mark certain parts of the recording, sort of like making a note to himself to pay special attention to that part of the interview later.

The third theory is that Blanc is using it as sort of an improvised lie detector. One meant to interrupt the interviewee and see if they could stick to their story or have to think up a new lie.

He presses the key whenever the conversation starts to get off track. It's to jolt the detective into getting back to business.

That particular sound of the piano is used in cinematic mysteries when the plot thickens. He's simply mocking the mystery genre by pressing this "suspense key" over and over when one of his culprits says something that might relate to the murder, or make them more guilty, as happens in mysteries.

Whilst there is no official proof, the fact that the names are soo close it is reasonable to deduce that Harlan Thrombey is an homage to Harlowe Thornbey.