I-94

If you visit United States (as non-immigrants), you will be issued I-94 card. This I-94 card must be returned upon leaving United States, otherwise your departure will not be recorded and you will be included in the list of over staying persons.

I-94 is a small green or white card given to all non-immigrants
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.

It'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:

ACS-CBP SBU

1084 South Laurel Road,

London, KY 40744

USA

To confirm that departure have been recorded, you may write at this address:

Customs and Boarder Protection

1300 Pennsylvania Ave.

Room 5.4.D

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.