I'm using the free "Meet Gavern" template from http://www.soluopen.net. The demo shown herehas a login link (top right) and results in a modal box with the login form. This is part of the template (seems hard-coded), it doesn't appear to be a module in the "menu" position and it's not a menu item that I can identify.
The template worked on my localhost (3.6.5) recently but after updating to 3.7.0 this problem arose - if you click on the Login, the modal appears for a brief second then disappears - Joomla v3.7.0 test site here: https://soluopen.net