On February 21, 2006, […] Hubble began seeing something brighten. It continued brightening for about 100 days and peaked at 21st magnitude in two near-infrared colors. It then faded away over a similar timescale, until nothing was left in view down to 26th magnitude. The object brightened and faded by a factor of at least 120, maybe more.

Basically they have no idea whatsoever what it was or what it could have been…

SkyandTelescope.com – News Blog – Hubble Finds a Mystery Object