Buy a cap keeper!
Drove up to Portland Int'l Airport to pick up my son-in-law and granddaughter this evening. Didn't notice I'd knocked the lens cap off my 5-15 when I took the camera out of my pocket in the car on the way up. It apparently fell out in the arrivals area when I got out to help load the suitcases. Grrr!
At least they're cheap. Ordered one from B&H for less than 10.