I'm not an expert so my apologies for any ignorance or "non technical" talk. I've an account on a website. I signed out and then revisit that website as a guest and they says "Welcome Back". I tried to chat with online agent and they know that I'm an existing user. (Although they didn't mention my user name etc but still they know that I often visit their site.) I clear all relevant cookies of that website and even restarted Firefox but still the same situation. Even I tried to use a VPN but same. Just curious how do they know that I've been visiting this website before. Thanks for help.