[tlug] How to detect unwelcome visitors on my macbook?

[Ulrike Schmidt <ulrike@example.com>]

I have a MacBook running Snow Leopard. Yesterday, when I was taking a 
little break from work and leaning back to think I suddenly saw a 
movement on my screen. It looked like someone was testing my mouse, but  
it lay motionless on the table.
I immediately stopped the machine and called my local Mac guru, who 
unfortunatley was not in. Then I thought I could probably at least 
finish my current work if I switch off the WLAN. I booted and switched 
off the WLAN as soon as possible, but thunderbird managed to start 
without my help. Then everything was quiet.
How can I check what happened? And after I have fixed my security 
settings (which have been rather lax), how can I check that everything 
is fine?
I tried google, but somehow don't seem to find the right search words, I 
only find happy MacBook owners ...
Uli