There's 33 of them in version 0.10. If you find one it unlocks a CG in the Extra Gallery in the main menu, and in the Hamster Gallery on the phone (top left button while playing). Those CGs are not relevant to the story but nice to look at.
If you're unsure what they look like, here's one for free: When the MC visits the crime scene for the first time and talks to Touka, there's a space hamster behind her near the window when the camera shows only her. Click on it and it'll unlock a CG of Touka.
Cheers!