Here's a guess: Perhaps you're operating on a system using address translation. This means you're sharing a few 'real' IP addresses with everyone else on your LAN.
Now if one of you co-workers is PPRuNing then your shared address is loggeg as having 'posted' recently.
To check you need to find your local IP address. Type 'ipconfig' on a windows system from the command prompt. Generally most sites set up like this will use the 192.168 range although you might see 172.16 etc.
Another possibility is that the person who dialed up before you (and who's address you've taken over) has just posted to PPRune. Unlikely but with 18000 users you never know.