That was me! If I'm alongside someone and don't see them looking in the mirrors, I honk to let them know I'm there. Many people do this too and I don't find it offensive or anything. Just a reminder you are not alone on the track. I've seen some people who think they are.
You don't need surround sound to determine the direction. Headphones or stereo speakers are enough.
Boy, do I wish it was so. Many drivers don't use them. I use them but does the person I'm next to? How can I be sure? Apart from the rare eye contact, when both look at the same time.
Don't confuse real life to a sim. All real life drivers are reguired to keep an eye on their surroundings and have the best Hi-Fi surround sound system available and have a full 360 degree fov on their wraparound monitor.
Yes, I agree with the overuse issues. But I don't see the harm in a single toot to let someone know you are right there.
Besides it's a good way to blow off some steam when someone nudges you.