Assuming that a miscarriage is buried in a permanent [>40 days] dwelling of a Nochri, even if he is a bachelor: Pesachim 9a
Above case, where pigs or rodents have access to the dwelling: Pesachim 9a-b
Entering a field where a grave was lost: Pesachim 10a
Where a body was lost in a field, and one searched and couldn't find it: Pesachim 10a
If a body was found, but one can't verify that it was the same body which was lost: Pesachim 10a