hmm, no, that's not so much about your browser storing your data. It's about sending data via insecure channels. And your browser doing this automatically as you decide to open the page the login belongs to.
It's simple to sniff all the data that is in the air. You just need to notice it, you are a passive receiver who is just plotting the bits.
Session Hijacking is a more advanced technic.
If you want to have a look at what's going on in your network or around you in the air - try it:
Wireshark - the most common network analyzer with decryption support and mostly all, what is needed ;-) try it, it's free.
Air Snare - a freeware tool to detect unknown visitors in a network
Microsoft Net Monitor - a freeware tool observing your own traffic.
regards
Torsten
