There was a book when I was a kid that I had. It must have been an early 2000s or a 90s book. It was about looking through this keyhole. I remember one page being peeking through the blinds to see outside. The last page was a pull out pop up with a keyhole cut out. You could see several different rooms through this layered keyhole, like a boy picking his nose or an old lady and that. Its art style was wacky and strange and the book cover was a watercolor grey with a keyhole or something.
The Snoops by Miriam Moss perhaps.
From the Publishers Weekly review:
Readers get the scoop through the eyes of the Snoops, until the binoculars are turned on the last page, which offers a 3-D peek into number nine Keyhole Place. There readers can spy the Snoops themselves, picking their noses, practicing karaoke and snarfing chocolates while clad only in underwear. Moss's (Jigsaw) text creates a hushed, conspiratorial tone and creepily paranoid atmosphere. Spidery, crudely rendered hand-lettered text sets off the eccentric style of French artist Durand's cartoon illustrations, often framed by a keyhole, raised blinds or a binocular shape.
Amazon reviewer mentions accordion-style pop up at the end.