Why is 'I'm on' used? Scent on? notice on? or is there an abbreviation for other verbs?
"Scent" is actually a noun here. The preposition "on" is used after the noun "scent" to refer to Shere Khan following a scent and trying to find the source. He uses "I'm on" to describe this process. This sentence could also be switched around to be "I'm on this scent". The preposition "on" has many different uses but can be paired with nouns to describe being on the surface, above something, or in the process of doing something. Ex: I'm on a run. Ex: I'm on the grass.