I understand the need to train my model with images that do not contain my ‘drone’ class as well as to try to have a dataset as close to the real world as possible.
However, I think my problem lies in the model, since unfortunately a detection model trains with a loss function for classification AND a regression loss function for bounding boxes. So for the examples without class (with only background),  I have to generate a bounding box.  For me the most intuitive way is to put a bbox the size of my image… It’s not ideal, but I don’t see how to train my detection model well without a regression loss function for my background examples.
Thanks again for your answer