I have faced an intriguing issue in SharePoint 2013 environment. After some user complaints we have found that some users in User Information List don't have any properties set anymore, except their login name. We don't use custom properties, but the most basic properties were missing - Email and Display Name. All affected users were from other AD forest/domain, which we have two-way trust to. Not every user from this domain was affected. The modification date of user item is the day when we started to get complaints. But 'Modified By' property is System Account, so it's not very useful for investigation.
We do use User Profile Service Application, but we don't have connection to that domain and never had. However, there are user profiles from that domain inside of UPSA database - they are empty as well, no properties except the login name. I checked FIM Metaverse storage with MIISclient tool, and there is no records for user identities from that domain.
This particular SharePoint farm doesn't have much customizations and no customization is related to user profiles. We don't use any external user identity sources, except AD domains.
We are able to fix missing user properties running
Set-SPUser -SyncFromAD cmdlet, but I want to understand why it happened and can it happen again.
My assumption is that for some reason user profiles were imported to UPSA database with no properties filled in, and than it was synchronized down to the site collections. But I'm stuck trying to prove it.
Can anyone answer to the following questions: 1. How is it possible to push user profiles to UPSA database without involving FIM service? 2. If UPSA is not involved, what other process/service in SharePoint can rewrite site collection User Information List?
Any ideas will be much appreciated!