Answer:
To be honest, I don't know the real answer to your question. It can be used as a noun, adjective and adverb but it seems to me that it's rarely used as an adjective. According to what I've observed, it's more often used as an adverb. Anyway, here's the sentence: As an adverb: He traveled incognito to escape his responsibilities. As a noun: His incognito was in a lot of danger.
Note: Correct me if I'm wrong. I would gladly accept corrections.