Hello! Thank you so much, we're glad to hear co-open was to your liking!
The last square on the second page of collection should be the only square with the star/diamond sign, so it's in a different location! Have you been to the topmost floor (as shown in the elevator)? You might need to do something to clean the button and make it work to reach the location, but once you're there it should only take a look around to notice something special. Hope this is of help, and good luck! Congrats on getting every other collection item too!