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HTTPS is for a secured transaction. If you do not go back to a HTTP it's by the webmasters design or they do not do proper clean-up after the transaction. Regardless it should have no real effect on your browsing. When you go to another site (different domain) the HTTPS will drop.
ToddN2000: Thanks for your reply. Does it not leave the https secure server open for the time being until the site is exited?