"This was an active/passive with no VMs running on the Failover domain."
VMs not running on one site doesn't mean they don't have components stored there - if you are/were using the default Stretched-cluster Storage Policy (RAID1 across sites) or any other Stretched-cluster policy then this isn't really 'active/passive' from a storage perspective.
From what you have described there are 2 likely causes, either you are:
1. Unstretching this cluster incorrectly and you still have data with Stretched-cluster policies applied - checking for such Objects just on the stretched Storage Policy(ies) is not sufficient as this only shows VMs/Objects in current vCenter inventory - check with the output from 'esxcli vsan debug object list' (--all required if 6.7 U3).
Proper process for unstretching a cluster:
2. Have some data with a Storage Policy of FTT=2,FTM=RAID6 - this requires 6 'standalone' nodes (e.g. not in Fault Domains) for compliance and thus if you only had 6 applicable nodes then you can't place one in Maintenance Mode with 'Full Data Evacuation' option as you wouldn't have anywhere applicable to move the data to (as all other nodes already have components and thus moving the data to any other node would violate the Storage Policy).