YSK: If you're on the phone with somebody (usually a company) and they ask to "put you on hold", it may just be their end that's silent, not yours, and they can still hear everything that you're doing.

My dad told me that when he was a hiring manager, they used to put candidates on hold to discuss something with the other managers in the room during the interview. He said a few times, they would put the candidate on hold and they could still hear what he/she was doing. They had people sneeze, yell at someone, fart, and flush the toilet while they thought that my dad couldn't hear them.