If you have ever visited Thailand, you must have seen the TM.6 card before. This card is given to you whilst you are on the plane, arriving into Thailand. You must fill out the card and submit it to immigration upon arrival.
The departure section of the card is then placed into your passport and checked upon your departure of the country.
If the departure section of the card is lost, you may need to visit the local police office to file a report. You can then get a new card from the immigration office or at the airport prior to your flight.