Around the lab I am always looking for my 3,4,5 hex key. Apparently I am not the only person that has this problem. Rehab Systems puts a kit of hex keys anywhere they think they might need/use them. The hex keys then never leave the specified area. Here is the little rig they made to hold the hex keys. They used a scrap piece of plastic and drilled some holes in it to hold the wrenches. Simple, brilliant, and now I know exactly where my 4mm is and will be.