Have you handled what to be done after the expiration of the session ?
Generally after session expired it should be moved to login page on performing any operation on page.See this configuration to work as expected :-
<system.web>
<sessionstate timeout="1" />
<authentication mode="Forms">
<forms loginurl="~/Login.aspx" defaulturl="~/Home.aspx" slidingexpiration="true" timeout="1"></forms>
</authentication>
<compilation debug="true" targetframework="4.5" />
<httpruntime targetframework="4.5" />
</system.web>
The page will automatically be redirected to Login page if you will try to do any operation after session expiration.
Please let me know if this works for you.