O.P.: Thanks for the heads-up on the beer bottle. Sounds like it was carefully placed to do damage, not just be hidden from a roving ranger in the park.
Years ago, we had made a shopping center stop, for grocerys. Came out of the store a half hour later, had a quick lunch, and were ready to get back on the road. I did my usual walk-around, and saw that someone had layed a shopping cart on it's side, and wedged it under the front of the motorhome. We couldn't see it from as we approached the RV from the store, and if I had just driven away, it certainly would have caused some damage under the motorhome.
I also always check the tow bar pins, and will now add looking between the duallies, before rolling.