bill, The phrase "Smell you later." was popular a few years ago. It has the same meaning as "See you later." It is a humorous way to say goodbye, an example of adolescent humor. The cartoon character, Bart Simpson, uses the phrase.

What is the rest of the sentence? if you can smell someone it could either mean that they have bad body odour or they smell fresh, clean with a hint of perfume or after shave.

It means I know what you are up-to or I know what you are planning.