When I got married my husband pretty much immediately showed up as my spouse on my transunion credit report as my spouse. How did they know that? Our names are different. At the time we didn't have any loans together. We lived together but so do siblngs and roommates. We didn't register for any wedding registries or send out any announcements. Our wedding consisted of signing some paperwork at city Hall. They also marked me as "Active Duty Military or Dependant" (hubby is in the army so I became a "dependant" when we got married). So the only logical explanation is transunion can access DEERS, but I would hope the DoD doesn't allow random private companies access to DEERS... They DO have a website where you can lookup if someone is covered under the SCRA but dependants aren't covered under the SCRA and don't show up when queried (I tried).
Again this is my credit report. I didn't report a change in my martial status to any of my financial institutions. Not banks, not credit cards, and we already had a joint account for two years before we were married.