On Windows build slaves 'p4ignore.txt' is used for P4IGNORE files based on the following code:
https://github.com/jenkinsci/p4-plugin/blob/2d15b39ddc1e03b901c844b330b93779c56ee856/src/main/java/org/jenkinsci/plugins/p4/client/ConnectionHelper.java#L170
This should either be configurable (if not already) or at least documented.