they wouldn't be putting up signs if she had left a note
Not really much point in putting up posters when, as far as they know, they're the last non-mutated humans on earth.
So the whole thing is a bizarre fever-dream... I have to say, Velma's subconscious is a pretty messed-up place. Freud might say some small part of her wishes it could turn off her conscience and finally embrace this horrible madness she hath, if inadvertently, wrought. She casts herself as a Queen of Hell, playing judge, jury, and executioner, the sole hand keeping the lid on this teapot tempest. Of course, as we see with dear brother Rufus T. Dinkley in subsequent issues, controlling these creatures is just the fond, fevered wish-dream of a madman.