cat that ate the canary

noun2

1. •
   A person who appears self-satisfied or smug, especially while concealing something mischievous, prohibited, or private.

   • He talked freely and carried the smile of the cat that swallowed the canary. — 1910
2. •
   A person whose appearance and behavior suggest guilt mixed with other qualities, such as satisfaction or feigned nonchalance.

   • Like the cat that swallowed the canary. Not only do you feel a little bit guilty 'cause you can't stop, but you feel pretty good, too. — 2001