People get sloppy. A Russian GRU operation in Europe was discovered (reported on within the last 6 weeks) because all of the operatives used the same address for vehicle registrations.

> All 305 names had passport numbers and, in most cases, mobile telephone numbers attached. All information can be found on the publicly-available automobile registration database.

https://www.telegraph.co.uk/news/2018/10/05/putins-spies-new...