Call. the theater
Call the theater first and see if they have a lost and found section. If they do, get down there to collect the keys or have somebody collect them. If they don't have that section, then you will need to get down there and talk to the folks at AMC. The keys are most likely removed from the screen you were sitting in since they clean them after every show.
I would suggest you go there in person and take a flash light. To try and explain what theater you were in and where you were sitting could be hard. They could be in your row our the row ahead of you.
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