By default when a zone updates it shows a yellow highlight that fades away. However, if the zone is updated before the highlight has completely faded then the highlight remains (until the page is completely refreshed).
To see this in action, go to the JumpStart demo application:
And double-click the link "Refresh time2 only - this is an AJAX-enabled ActionLink".
(Although JumpStart is currently based on T5.0.18 we see the same behaviour on T18.104.22.168)