my experience from sessions
is that there is 2 things that removes session variables:
1. close browser
2. script unset value or destroy session
There might be a session lifetime value in php.ini
that would delete very old session files.
But this is sure not so in your case.
Otherwise, I can not understand other, than that SESSION value still exists
I can not remember I ever seen anything else.
... well, I have at times, deleted session-files from my session-folder
not to need to close my browser, when doing some code testings
Regards 🙂