I once had a battery swiched. My good battery for someones cracked junker. It happened at either a campground or a local dealership where I had my yearly free checkup. Dealer brought it to my attention and as soon as I looked at the battery, I knew it wasn't mine. One thing, this battery was exposed to the elements (dirt road dirty) and mine didn't have a blue top on it. Can't say for sure when and where it happened, but it won't happen again. I bought a metal strap with eye for a tamperproof lock.
2012 Flagstaff Micro Lite 21 FBRS