Do you get a bar at the top, something like "Noscript blocked a META redirection...." ?
What Google-related scripts are you allowing, and which are in your permanent whitelist?
Things like igmodules, that aren't familiar to users of basic Google services?
When the page reloads after a permission, often there will be new entries in the NS menu, as the newly-allowed scripts call to yet more script sources. Annoying, but a growing trend on the Web. (sigh).
So it may take several series of allow or TA, refresh, look at the menu again.
If you're a frequent user, you might want to put *all* of these in your whitelist, or use ABE
to create a rule which allows these scripts at Google sites, but nowhere else (unless there are other sites at which you want to allow these scripts, and these sites, too, can be included in the rule).
Do you use NoScript's Advanced > Force HTTPS on cookies, on behavior, or on both? Neither?
Certain that cookies are not being dumped when you leave your page?
Next time, before you leave the site, look in Firefox Tools > Options > Privacy > Show cookies, then leave the site for somewhere else, and see if the same cookies are still there. Unlikely to be a NS issue, but a common cause of unwanted logouts is intentional or unintentional deleting of the site's cookies. (Do you use a third-party cookie manager?)
If none of this gives a solution, then the next time it happens, please open Firefox Tools > Web Developer > Error Console.
Click the red X. Copy and paste any Error messages here.
Then click the blue i. Copy and paste here any Information messages that are pertinent to Noscript. Typically, they will start with
[NOSCRIPT], [NOSCRIPT XSS], [ABE], or similar.
Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:18.104.22.168) Gecko/20111212 Firefox/3.6.25