Maybe it's pre-installed on a Nexus? This is how it has worked for me on Motorola phones, and my Samsung tablet. I can log into the ADM website using my Google account, and can see my phone on a map, as long as the Location settings are On. But I cannot lock it, make it ring, or erase it unless the ADM app is installed, opened up once and given Device Manager permission. Once the ADM app has Device Admin permissions, I can perform the remote lock & erase functions. Maybe that's not how it works on all phones, but it's how it has worked for me. And...when I logged into my account recently just to see what happened when I selected my tablet and selected Find from the website, my tablet notified me that someone was using the ADM website to find it. The app also helps to let you know if someone other than you is trying to access your device remotely using the ADM website.