Crossword Genius

Police dogs are trained to detect this number tag (4,4)

Ross

I believe the answer is:

hash sign

'number tag' is the definition.

I cannot understand how the remainder of the clue works.

(Another definition for hash sign that I've seen is " #".)

I've seen this clue in The Guardian.
Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.