Why did the Mechanical Hound sniff around Montags house?

The Mechanical Hound sniffed around Montag's house because it was programmed to detect the scent of individuals who violated the law, specifically those who possessed books. Montag's growing discontent with society and his secret collection of books made him a target for the Hound. The advanced technology was used by the authorities to enforce conformity and suppress dissent, thus ensuring that Montag's rebellious activities did not go undetected.