What does The Batman's post-credit scene mean?

All we see after the credits is the Riddler's question mark text cursor pop up on screen, followed by the words 'Good bye'.

Then there's a flash of images, including what looks like a URL. We'll have to get the chance to slow the footage down for sure, but for now we believe this is simply the URL for El Rata Alada, the Riddler's site that plays a prominent part in the film - and featured in the movie's ARG marketing campaign before the release.