Yeah, my bichir has done this since I've had him, except his spot is sitting on the top suction cup of the filter... the only other fish are 4 med. tiger barbs and 3 4-5" botia kubotai.. and I've never seen them bully him once. I've got red cabomba covering the back of the tank, some tall-ish swords in the middle, and banana plants/java moss up front with a few crypts, so theres plenty of cover for him to hide in if he wants to.
I think it could be: a) being bullied a little bit b) they like to have not only a hiding spot but also simultaneously a good vantage point so they can see everything going on around them, and c) when mine doesn't know I'm there, he's out and about. As soon as he notices me, up to the heater spot he goes (granted, he's pretty new and only 4"... my old one did the same thing until 7-8").
If he's new/smaller/younger, I'd think it's most likely you scaring him. If he's a big guy, then he shouldn't be scared of you anymore.. you might want to consider moving whoevers bullying him (EDIT- I interpreted "only for 3 days" as "the past 3 days".. if you mean "3 days since you got him," then I'd shoot for option B.. hiding places etc.