I use or have used linkedin contact listpk to maintain track of former and prospective business associates. If you know one name, you can find others and their employment as most folks do not properly secure their accounts. Other online sources are public records maintained by various government agencies. Even old fashion sleuthing at an employers parking lot can yield license numbers that can be traced back to individuals. Throw in data mining software and you can come up with a surprisingly accurate list. Bottom line is that in the internet age, you cannot be anonymous.
Probably happened that way.