All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
The scholars stated that if a dead or killed person is found and there are some signs that prove that he/she is a Muslim, like circumcision, wearing clothes peculiar to the Muslims, having dyed hair with henna, having pubic hair shaved, then in this case, he/she is treated like a Muslim, he/she should be washed, enshrouded and buried in a Muslim graveyard, whether he/she is in a Muslim country or a non-Muslim country.
However, if there are no such signs on the dead person, then if he/she is found in a Muslim country, he/she is treated as a Muslim, and if he/she is found in a non-Muslim country, he/she is treated as a non-Muslim, because in principle a person who resides in a country, he/she is from that country, and the ruling which applies to the people of that country applies to him unless there is evidence which proves the contrary.
Allaah Knows best.