He paused outside the doors, taking stock of his men, careful not to give any sign of his thoughts. (source)
My understanding is the phrase "careful not to give any sign of his thoughts" is an adverbial modifying the main clause. In that case, shouldn't it be led by the adverb "carefully" instead of the adjective "careful"? If I am in the wrong and the original sentence is correct, what kind of phrase is "careful not to give any sign of his thoughts"? What is its grammatical function? And why can "careful" as an adjective be used here?