It's worth pointing out that it's very easy to leave the fuel filler cap off an R22. I've seen people take the cap off, dip the fuel, then put the dipstick back under the seat and forget to put the cap back on. It sits there right next to the hole until you take off, and bye-bye filler cap. I had a friend of mine (ahem) do that.
Early models of the R22 had a chain attaching the filler cap, but it was soon discontinued. The reason I heard was because if you left the cap off it would be ripped away in the cruise and might foul the tail rotor. Far better to have it fall off on take-off.
That said, there are a lot of light-fingered bastards about. The helicopter museum at Weston used to allow visitors free access to their exhibits until the instruments started going missing. People used to turn up with screwdrivers to do a proper job.