Lost green card in my house! I need tips on how to find it!?

My father lost his green card in the house. He told me that he put it in a certain place. We have looked everywhere in the house! He said he never left the house with it since it came in the mail! I need tips on finding it AS SOON AS POSSIBLE!! I wanna suprise him and go to Montenegro with him. We have family there and he hasnt seen his family in FIFTEEN YEARS!!!!!! I really want to surprise him and go with him on this highely important oversea trip this summer! BEST ANSWER 10 POINTS!