what is running VSS on my server

Client's SQL Server farm DBs are backed up by third-party tool that does **not** use VSS. About half of the instances show backups being made by some tool that **is** using VSS. I can disable SQL Server VSS Writer to stop these errant backups, but how do I track down what is running the VSS backups? **keywords**: Volume Shadow Copy Service, volume-shadow-service [I don't have rep to create tags]