Bizarre Popup Requesting Username+Password

Hi. I wasn't sure where to post this because I have no idea what the cause could possibly be, but recently I and other members of my forum have been getting a really weird popup in topics asking for a username and password for something called "tescotechcentral". I've tried searching through the coding as I've edited the templates, css and widgets, however I've found nothing out of the ordinary and I don't know how I could've even done something like this on accident. For most, it seems to only pop up in topics although one person said they got it on the portal as well. Most of the members know not to put their information in but it only takes one confused person to do so lol. Below is a screenshot example. Any help?

Spoiler:

Hello,
Try to disable first time templates, after that announcements, and javascript. If this popup dissapears, notify us!

There could be a code in the posts that is causing this pop-up. If you have HTML activated, turn it off and see if the pop-up appears.