This post by Darla Mack made me remember what I used to do with my own smartphone.

Most smartphones and PDAs that feature removable batteries have a tiny space just between the battery itself and the cover. Just enough to squeeze in a Php 500 (around $10) bill, spare ID pics, a secret note to myself, or even a small map. A secret compartment that's relatively safe.

Some of my friends also place picture stickers on the battery. So they can see it whenever they have to remove the battery to change the SIM (a more common occurrence over here than you may think).

Do you use this secret compartment at all? If so, what for?