The iStore session timeout parameters are outlined in document -
11.5.10/ R12: How To Control Session Timeout in Oracle iStore? [ID 377436.1]
The Guest user is used as the default user when accessing iStore before the user logs in.
Only the actual iStore username that is logged in should be displayed in the Profile.
Please be sure you are on the latest CPU patches for R12, see the latest CPU document -
Oracle E-Business Suite Releases 11i and 12 Critical Patch Update Knowledge Document (April 2013), My Oracle Support Note 1530756.1
Note that if more than one user logs into the same PC, then the user must be sure to choose the Logout button -
How to avoid interference between Sessions / Responsibility Changes and Login / Logout when more than one browser is open [ID 365505.1]
Please open an SR if random user names are appearing in Profile.