Something appears not to be localized

I am creating a completely new translation/localization for OpenCart and I seem to have found a string that is not localizable. On the other hand I can't find the string at all when I search for it in all files under Windows.
(creating the localization is quite time consuming so I can't say for sure that the string isn't in the language files, but I can't remember seeing it anywhere)

The string appears when you look at the picture of a product and you enlarge it. Down to the right side there is a text (which partially is a link) saying: "close or Esc Key".

Any ideas where this string is located?