they could easily have left it locked.
How? If they took their lock (which, as you say, isn’t in the picture) how were they supposed to leave your bike secured? Basically you have had a scare, been lucky so learn from it, park your bike where its supposed to go rather than where you want to put it.
If you are worried about a bit of bird poo why not get a tarp zip tied over the cage roof or something. Seems you are looking to blame someone else for your own incompetence.