Are you using Forms authentication?
Forms authentication uses it own value for timeout (30 min. by default). A forms authentication timeout will send the user to the login page with the session still active. This may look like the behavior your app gives when session times out making it easy to confuse one with the other.
<system.web>
<authentication mode="Forms">
<forms timeout="50"/>
</authentication>
<sessionState timeout="60" />
</system.web>
Setting the forms timeout to something less than the session timeout can give the user a window in which to log back in without losing any session data.
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