Entered USA by land, left by air, green paper still stapled in passport, do I risk something when going back?

Question

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?

Answer 1

You are supposed to turn it in to the airline, but it is no problem if you didn't. The US Government states (https://help.cbp.gov/app/answers/detail/a_id/752/kw/I-94%20was%20not%20collected, second paragraph):

[...] If you departed by a commercial air or sea carrier (airlines or cruise ships), your departure from the U.S. can be independently verified, and it is not necessary to take any further action, although holding on to your outbound (from the U.S.) boarding pass - if you still have it - can help facilitate your reentry next time you come back to the United States. [...]

I recommend to staple the outbound boarding pass right onto it (and keep it), so you have it if ever needed; otherwise you should be good.