1) I can't give you a 100% accurate answer on this, but another Orgah probably can. I do not think that Steam remains as a background process once you close it down however.
2) Steam searches for auto-updates everytime you connect to the internet and have steam running, in much the same way Windows update does. If Steam is running in offline mode it will not search for updates and will launch games just fine. Once Steam has found updates it will install them automatically (providing it is running). It isn't a background thing, it's part of the main program. The updating process is usually very quick, unless Steam has not been updated for a long time. It interferes very little with running games.
3) Steam does not leave hidden files and it doesn't mess with low-level system files. Steam is definitely not the same as SecureRom. It's a 100% clean uninstall.
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