Most eye opening thing I've had happen on the slab has happened twice now, a diesel exploding a rear trailer tire right in front of me. Thankfully in a car both times, would probably have been ugly on a bike.
I too stay mostly on the backroads when I'm on the bike (no corners on the interstate). But there are things there too. Deer, turkeys (turkey at chest level will take you right off the back of the bike at 70mph), buzzards, and the carcasses they are eating in the middle of the road. Gotta always keep your eyes open.