I have suddenly begun to get these pop-ups on my 27 inch iMac, which doesn't move -- but I do have a Linksys Wi-Fi router which has a "Guest Network" requiring a guest to login.
The answer is -- it is an undocumented "feature" in OSX.
https://grpugh.wordpress.com/2014/10/29/an-undocumented-change-to-captive-networ k-assistant-settings-in-os-x-10-10-yosem…
"A captive portal is a network that forces an HTTP client to see a special web page (usually for authentication purposes) before using the Internet normally. A captive portal turns a Web browser into an authentication device. These are commonly used on wifi networks where authentication to the private network is done via a login browser page, rather than via the use of a WEP or WPA2 key, for example in some coffee shops and airports, and hotspot providers such as The Cloud and ATT-Wifi.
On Apple devices, if a captive portal is identified, a special application in /System/Library/CoreServices called Captive Network Assistant.app is opened. This is a very limited browser, separate to Safari, with no address bar or navigation buttons. . . "
See also: http://apple.stackexchange.com/questions/45418/how-to-automatically-login-to-cap tive-portals-on-os-x
And: https://www.reddit.com/r/applehelp/comments/3dc6jv/strange_popup_every_5_minutes _regarding_my_wifi/