@Stu: Do you hear the light crash to the ground? I have various Cat Eye & Planet Bike lights and the only time I've lost one was because it bounced right off the cable tie i had it clipped to on the back of my milk crate when I hit a pothole. I knew right away because the plastic casing, (filled with batteries) hitting pavement and bouncing apart was loud! Can you definitely rule out theft?
RE the bell: mine sounds different when it's wet but otherwise seems quite sturdy. It's a $10 "incredibell." maybe try a lighter hand when drawing back the plastic spring?