Are you certain that Adblock is disabled? What browser do you use?
I'm using Firefox(FF) and this is what it should display(ABP is the icon for Adblock on FF):
AdBlock enabled(Red ABP):http://prntscr.com/716rnl
Adblock disabled(Gray ABP):http://prntscr.com/716tge
If it is disabled and you still see that popup, it might just be a case of some network packets getting dropped(and they happened to be the ad related ones). But you shouldn't see it consistently though.