During a previous trip in North America, I entered the United States by train from Canada and left the country by plane.
At the moment we crossed the border by train, US officials came on board to inspect passengers at their seats and had me fill a green form although I have a pending ESTA. As a result, a stub of that form was stapled in my passport. I remember this process being common at airports a decade ago, before the existence of the ESTA.
When boarding my flight to leave the country, no one ever checked my passport except at check-in and security for ID purposes. Now, back home, I find this little green card named "Departure Record" still stapled in, stating that "failure to surrender it may delay further entry into the United States" or something similar. I thought that card was to be taken away at some point in the airport.
Do I need to take any action if I want not to be seen as suspicious on next entry? Or, can I safely remove the card from my passport?