Customs & Immigration forms

Spent two weeks in late May and left Zihu in early June. When landed in MX City customs stamped passport. Returned from Zihu to Mexico City for layover to states. Landed in Detroit . Went to custom. Handed officer my passport and was told to look into camera. Officer handed back my passport and said thank you. Put suitcase on conveyer. Waited for layover for flight home. Absolutely no form to fill out whatsoever. Was never given a form. Was never asked to fill out declaration form.

To tack on a bit to this thread:

As a Residente Permanente do I have to go through the procedure when leaving Zihua- meeting/finding the Immigration Officer at the Airport and having him/her 'officially' sign me out?