PayPal Observation
I have noticed this whether or not there is a "go to my PayPal account" link in addition to the "return to seller's site" link after the payment has been processed. Even after going to the seller's site, I can open a new tab, put in the PayPal address, and be in my account without having to log in again.
I know the login is supposed to time out, but the fact that it does not exit when a transaction is complete is a bit unsettling.
Another observation is that if there is a "return to seller" link, that link needs to be used to complete most transactions fully. I now make a habit of opening the PayPal link in a new tab to ensure the account is logged out once the transaction is fully completed.