Some browsers sometimes do that, another instance is running, even when you know you have shut it down on a previous session (login..)
You can always run top
if you have that installed and see what processes are active..
--and you could check the start-up services to see all that you have running
--for example avahi, unless you always want to be scanning for any new local devices (network cards, printers etc..) isn't really needed: once you have a system running the way you want
And some other services may also be excluded, such as automatic updateshttp://nixcraft.com/getting-started-tut ... nning.html
--you can always check manually for updates
--there is a GUI selection via the vendor desktop for that, but the link is for another method