There isn't a "simple" way, but for a one-off, use Postman. Naturally you'll need to create a LaunchPoint service, API role, etc.
You can go to marketing activities/campaign inspector and search "delete" to find the smart campaigns deleting leads from Marketo.
This is what I was going to recommend as well, Jeremy Foote. If you haven't used Campaign Inspector before, it's located in the second tab on your root folder in the Marketing Activities section. Ideally you don't have a lot of smart campaigns with the Delete Persons flow step in them so this should narrow your search considerably and it'll be a lot easier than working through REST API commands... unless you think that the delete record is coming from an API POST. Does that help?
It sounds like these contacts are deleted from your followup comments. Depending on your support contract and ability to direct support in the right direction they can assist in recovering them. This is an alternate approach to using the REST API that we have employed in the past.