I have a blank I-94 lying around home.... this is what it says on the back of the card:

IMPORTANT: Retain this permit in your possession; you must surrender it when you leave the US. Failure to do so may delay your entry into the US in the future. You are authorized to stay in the US only until the date written on this form. To remain past this date, without permission from DHS authorities, is a violation of the law.

Surender this permit when you leave the US:
- By sea or air, to the transportation line;
- Across the Canadian border, to a Canadian official;
- Across the Mexican border, to a US official.

Students planning to reenter the US within 30 days to return to the same school, see "Arrival-Departure" on page 2 of Form I-20 prior to surrendering this permit.

With airlines, they do have the capabilities to track when you left the country....but whether they do so proactively or want you to prove to them is another story.