Thanks. Sadly it was blank - but it turns out it's something in Azure doing snapshots (not found where it's putting them or if they are actually any use  -...

Ah - found https://docs.microsoft.com/en-us/previous-versions/sql/sql-server-2005/ms189548(v=sql.90)?redirectedfrom=MSDN

(via https://www.sqlservercentral.com/forums/topic/snapshot-backups-and-transaction-logs!)

So now to find which third party tool is doing it...