Assuming it auto-connects when an known WiFi network is found:

In that case I would install a firmware on my (Wifi) router that allows me to see the outgoing traffic. (for example: Tomato)

-check internal ip assigned to N900
-check to which internet ip this internal ip address establishes a connection whenever it does "it" again.

From identifying the destination you may find the related program.

Besides, I noticed that N900 keeps a connection to IBM lotus life. There is a thread about it somewhere but nobody knows why it does that.