Wierd circumstance.! HELP.?

Hiya all.

Can anybody advise as to what may have happened to my Nokia 5800 handset after it had been in for repair.
I received handset back yesterday and after setting up my profiles etc i then went to log onto Facebook, it was then that i noticed that my handset was allready logged on to both Facebook and Twitter under another user name, both of which are active and i could browse their pages.
I have since contacted the repair centre and they have asked me to send it back to them because there is a privacy issue to which they need to look at.
Now my question is, what is the posibility or my handset being used by someone at the repair centre or another customers details being uploaded onto my handset, and what you would reccomend i do next.?
Many thanks for your time and advice in advance.
Regards.