How does Twitter.com logout users after cache clear?
I noticed something very interesting this afternoon - when I clear my browser cache and cookies, Twitter immediately signs me out.

I assume they are polling for an authentication cookie every second or so via Javascript, and signing out users if the cookie is missing. Any other guesses as to how this is done?

Since this is my first encounter with this behavior, I'm wondering how common it is, the best way to accomplish it, and any security concerns one should have when implementing it (if any).
How does Twitter.com logout users after cache clear?
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I noticed something very interesting this afternoon - when I clear my browser cache and cookies, Twitter immediately signs me out.

I assume they are polling for an authentication cookie every second or so via Javascript, and signing out users if the cookie is missing. Any other guesses as to how this is done?

Since this is my first encounter with this behavior, I'm wondering how common it is, the best way to accomplish it, and any security concerns one should have when implementing it (if any).