User blog comment:Omega natsu2/Chapter 368: The Third Seal, Predictions./@comment-5268982-20140119103129/@comment-7418318-20140119111822

I did comment about it.

I theorised that maybe Silver intentionally let Happy escape, under the condition of not revealing his connection with Gray, either out of some compassion, the guise of compassion, while he really had something sinster planed with Happy or he simply didn't care for a lowly cat, so as long as Happy promised to not reveal the secret he simply couldn't be bothered with the neko.

As for the possible sinister plans: Silver might be using Happy as an unwitting listening device, tracking device or maybe even a "trojan horse" that will unleash some destructive spell on the guild when triggered or maybe even as a beacon to teleport troops right inside the guild.

Another possibility is that he let him go because Tartarus expects Happy's information will make Fairy Tail come after them and the Demons will be ready and waiting in ambush.

Considering how Natsu had a short moment of intelligence, Happy might have kept quiet about Gray and Silver's connection for the time being because he wants to tell Gray in private and let him decide how to handle the matter. Very unlikely considering Happy's personality, but not completely impossible.