I've actually done it twice during the first month after buying the RGC. My wife was with me on her bike on both occasions. The first time it was only a few miles away and it had been given to an attendant. I told my wife "I won't do that again". The next time I was 60 miles from where I'd left it at a Costco near my house. Imagine the look on my wifes face when I told her I'd done it again. It was a wknd and I wasn't going to make it back to Costco before they closed. I ended up buying a second cap from a nearby HD. The next morning I went to Costco and was given back my original cap. The original now sits in the saddle bag. Instead of setting it down on the gas pump I now put it on the right floor board. That's been working since the first few weeks of ownership. I think the key ring and hanging it off the brake is a great idea.
2012 FLTRX aka Black Sassy, Vivid Blk, Wind Splitter Windshield 12" drk, Vance n Hines Pro Pipe 2-1, SE Heavy Breather, Power Commander V, WO612 Outlawz Bars.