Logout Weird Prompt: Why is there a prompt when there shouldn't be one?

When I log out from some of my school accounts (sort of an intranet for school members), I occasionally get a prompt saying "Damn, I'm good!", or I might occasionally be diverted to a page where it says "logging out". Everything is normal apart from that. I can close the prompt and see the logout page.

In the past few years this has never happened, until quite recently. The prompts don't always appear, and they don't stop me from logging out, but this prank-like prompt looks quite fishy. Is it a bug/virus/worm/hack? Should I do something about it, and if so, what?