If you notice a particular user who, each time they login to their account, appears in a new location with a different IP address from their last login, is that not an obvious sign of VPN usage?
This is not entirely correct.
Most people are using dynamic IP address which make your IP address keep changing even you're didn't move to anywhere. Let's say that you're live in Los Angeles, but your IP address could show you're in Washington, Dallas, Boston etc.
Who is doing the assigning of the dynamic ip address, the isp or the router? Because one article I read elsewhere said the ISP, the other said the home router. Can a home router also moonlight as a DHCP (Dynamic Host Configuration Protocol)?