tense

Your sentence should be corrected as follows:

1. I think when this letter was found by you, you have come back from NY already.
EXPLANATION: I use passive voice for --WHEN THIS LETTER WAS FOUND BY YOU......

2. I think when you found this letter, you have come back from NY already.
EXPLANATION: I use active voice for---WHEN YOU FOUND THIS LETTER....

3. I think you have come back from NY already when this letter being found by you.
EXPLANATION: I use phrasal words for ---WHEN THIS LETTER BEING FOUND BY YOU.

REMARKS: YOUR SENTENTCE IS NOT CORRECT. e.g. you should use COME instead of CAME for present perfect tense.----COME, CAME, COME etc. COOK, COOKED, COOKED, etc. SEE, SAW, SEEN, etc.

Incorrect
1. letter found you? Letter 係死物,不能found you
2. you (have COME)

correct