What kind of images/objects are the most easy for neural networks to detect?
You are describing the field of robust object detection.
It depends greatly on the specifics of the data and model. Similar to person looking for their keys at night only under a street light, most robust object detection research is related to autonomous driving.
The simplest answer might be something out of sample from the training data. If the marker object is 100% synthetic, a model would quickly learn it since there would be no confusion with other inputs. Again, the definition of 100% synthetic would depend on the training set.