I-94 is a small green or white card given to all non-immigrantsIt's a long a story but we failed to return our I-94 card. As soon as I found out about it, I immediately search the net what to do. If you fail to return your I-94, you must send it back along with documents that can prove your departure from United States (e.g. boarding passes and arrival stamp to the passport) at the following address:
when they enter the United States. The I-94 card serves as evidence that a non-immigrant has entered the country legally. It is stamped with a date indicating how long the non-immigrant may stay for that particular trip. It is this date--and not the expiration date of the visa--that controls how long a non-immigrant can remain in the United States. A new I-94 card with a new date is issued each time the non-immigrant legally enters the United States.
1084 South Laurel Road,
London, KY 40744
To confirm that departure have been recorded, you may write at this address:
Customs and Boarder Protection
1300 Pennsylvania Ave.
Washington Dc 22209
Attn: Passenger Operations
For more detailed information visit this site.
It's a tedious process. Make sure that you return your I-94 card when leaving United States.