Black upper left corner of screen iPhone 4

Hi All,

I got my uncles iPhone 4 as a gift (he switched to 4S). At first everything worked fine, but about a month later I noticed, in the upper left corner, just next to the signal bars going black – like a small half-dot. I do not recall dropping the phone, and I keep it in a case since the first day I got it. The funny thing is, the spot/smudge, whatever it is, starts disappearing if I, for instance, read using Stanza for more than 5 minutes. Is this light leaking, or damaged screen (if it's damaged, how come the smudge disappears?). It's not something that is impairing my ability to use the phone, just aesthetically insulting

I searched around the forums and the web, but found nothing similar. And unfortunately, going to Apple to replace this is not currently an option, as no official Apple service exists in my country.

All thoughts and ideas are welcome.