An uncollected I-94, Arrival Departure Card becomes the responsibility of the visitor. Not having it entered, or properly recorded in the Customs and Border Patrol System could result in secondary inspection, delays and potential denial of entry during a subsequent visit.
At a minimum, the card and information should be sent to:
ACS – CBP SBU
1084 South Laurel Rd.,
London, Kentucky 40744
The I-94 card should not be returned to a consulate, embassy or even a local CBP office. The CBP does not acknowledge receipt.
 
Therefore, it is important to use a verifiable form of delivery (certified mail return receipt requested, or Secure Delivery through an authorized Federal Carrier like UPS). It is imperative that the applicant provide ample proof of the actual day of the departure and not just proof of the departure itself. Among the kinds of documents to prove the actual day of departure, CBP suggests sending a used airline ticket or boarding pass, a passport stamp showing date of arrival in home country, pay slips from employment outside of U.S. subsequent to departure, credit card slips showing purchases made outside of the U.S. post departure, etc.
In addition, we recommend that when returning the card the applicant include a statement (preferably notarized) on boldly colored paper identifying clearly the date of departure and requesting that it be entered rather than the date that card was received by the processing center. In addition, carry copies of the evidence with you when visiting.
To determine whether an I-94 has been received and the date of departure correctly recorded applicants should submit a FOIA request to:
CBP’s new FOIA office at the following address: U.S. Customs and Border Protection, 1300 Pennsylvania Ave., NW Attn: Mint Annex Building, FOIA Division, Washington, D.C. 20229.
The request should seek verification of the recorded date of departure as well as a copy of the returned I-94. The CBP does not indicate how long these requests are likely to take. Expect delays.