I live in Hong Kong and in the more built up areas, people honk their horn as soon as the traffic stops moving. It's not uncommon for me to be woken up by someone holding their horn while a minibus unloads it's passengers which is sometimes a few minutes.

People are often quite apologetic when you call them out but the automatic instinct is definitely to use the horn rather than the brake.

Agree with that, and that is interesting. Replied below to a similar comment. You also have to consider that infrastructure is far less developed in Mumbai, though.