Joani and I have been seeing these tiny magnetic caches popping up around the bay area. I think the first one we found was on a sculpture in San Mateo county. We asked around to see if anyone knew where these came from and the only answer was "you can get them off of e-bay". Finally, Kusanagi figured out what they were and gave me a couple to hide. They are marketed as LED jewelry, and use the magnets to attach themselves to clothes, ears, etc. Joani did her camo job on them and added a tiny log sheet. Bring your own pen! Here is a sample before and after picture:
Note: This is just a sample picture. The actual cache may look different.
One nice side benefit is the batteries they come with. They fit my Carabiner flashlight (it was going to cost me $12 to buy the replacement batteries at Radio Shack. Yikes!). I'm sure they fit other things powered by button cells.