I am not familiar with WMS specifically, but a little bit that might help: it's likely a custom field on a custom record instead of its own custom record. Annoyingly, the field IDs of fields on custom records also have the custrecord prefix.
As for how to find it...good question. I don't think you can pull that info up via Saved Search, but if you're receiving a PO, I'm guessing it's some subrecord of the Item Receipt? I'm guessing there can't be tons of those right? You might just have to open them up individually and look for that field.