there are several long threads on this topic. It looks like a patch line to the paired data in or out pad under the battery. I remember people adding a dip switch to short them for dumb chargers and inductive chargers.
try this one:
talk.maemo.org/showthread.php?t=37107
Anyways I would suspect either a shop repair or it is a Chinese 'new' refurbished N900 with a new plastic case.