There isn't much documentation on this feature. Best I can tell from the source code is it updates your session time with every activity you do on the forum (click a link, post something, etc.). If that activity time is within 15 minutes and you make an action, it keeps the counter going. After 15 minutes, it sets the clock back to 0. This is separate and distinct from your Session Timeout for the password cookie.
For example, if you clicked on a link at noon, then clicked on another link an hour later, you'd have about 1 second of activity logged and registered (time it took the system to display the next page, minimum is 1 second). If you clicked on a link, and then 10 minutes later clicked on another, it would register 10 minutes. Make sense?
As to the member ID number, nobody owns #1. That is blank. I might do up a database update to move saint to that spot, but we'll see.

#2 goes to an inactive account by _gyruss_. The honor of #3 goes to an active member... 1UP.