Ladder on Runway
It's obvious, isn't it ?
Passing aliens had a technical problem with their spacecraft. Needed to land somewhere so that one of them could get out and have a look at it properly. Alien in question was not that tall, so needed a step-ladder to climb up and have a look. Fixed the problem, forgpt to load the step-ladder into the spacecraft, and then took off back into space.
Since aliens put the ladder on the runway, all earthlings will be justified in saying it wasn't them