I thought i had the same 'problem' with mine. How is your lighting? I had kept the lights down for a period to help combat algae and it made my betta lethargic. I turned them on again on regualar cycles and he seems to be active again.
Also is the tank on a busy spot? Does he have contact with anyone else? If not weird as it may sound, he has formed an attachment with you and when you are not around he just finds a cozy spot to chill out till you come. After all he explored the tank and has claimed it as his (assuming there is nothing else in there).
At least that's the same behavior mine exhibited and all those the reasons for his 'eccentrics'.
EDIT: Try a laser pointer around (dont point him directly). If he merrily chases the dot and tries to bite it while flaring at it, he is simply bored and needs your company or some stimulation while you are away.
Last edited by Pilot00; 09-08-2012 at 02:43 PM.