Strange "download" brown tab appears in my webpages - Malware, Spyware, Virus...?

After 22 years in Windows, I decided to move to Mac a few weeks ago.

My new MacBook Pro Retina has OS X 10.92 and Windows 8.1 in Parallels 9.0.

95% of Software is for Mac and just 5 apps are in Windows.

Yesterday, while checking my credit card website, a strange brown tab that reads "download" showed over the logout tab, it appeared and disappeared with the mouse movements. Any time I wanted to logout, this new tab showed in that place. I did not click. I logged out just closing the window.

So far, this new tab just showed in this credit card webpage, that I checked is the original bank web page. See the screen capture #1: the brown tab at the right that read "download" does not belong in that webpage.

I downloaded Avast Antivirus for Mac, and after 8 hours, it found and removed an infection (I don`t remember the name, but it was something like "win32 adware"). I uninstalled Avast because it kept blocking my email.

I downloaded ScanVax for Mac and after a few hours it did not find any infection.

I have Norton Internet Security in Windows 8.1 (in Parallels 9.0) and it found and quarantined "Adware.Adpopup".

But the strange brown "download" tab keeps showing in the same place any time I open the credit card website. If I move the window, it appears again over the logout tab after a few seconds delay.

Until a few minutes ago, it didn´t show anyplace else, or any website else. It does not show if I open the same credit card wbsite in Windows 8.1 in Parallels. Just in Chrome & Safari in Mac but now...

While writing this question, the tab also appeared in this web page. See the screen capture #2: the same brown tab at the right that reads "download".

I was avoiding this bank website, but now looks like it is showing in more webpages, like this Apple Discussion webpage.

I already checked for bogus apps like Mac Defender, Mac Protector, etc. in my apps and looking at the Activity Monitor.

Any idea what is the reason and how can I handle it?

I really appreciate any help and thanks in advance.

Screen Capture 1: Credit Card Wesite - look the "download" brown tab that does not belong there.

Screen Capture 2 : This Apple discussion webpage - same "download" brown tab.