Just a few "hints"

#1 Next to some melons, by the food truck...
#2 In the pizza oven...
#3 Complete the dark maze and step on the pressure plate to unlock the door to it...
#4 In the club bathroom...
#5 On the Office Table of mouse traps...
#6 Above the screaming votekick soldier in the wooden hole...

Once you have collected all, you can visit the secret dev room outside your spawn, by the two soldier statues, which features silly powerups on the pink pillars, giving you 60 seconds of temporary powers, and art made by me, along with a pizza box that takes you to the skybox!

Enjoy!