When do you consider sciatic nerve imaging in cases of foot drop without clinical or EMG involvement beyond the peroneal nerve without clearly indentified compression site?

I think it would be reasonable to image the sciatic nerve in this scenario. Many sciatic neuropathies, especially those caused by compression or inflammatory (vasculitic) lesions, tend to affect the peroneal division much more than the tibial, therefore clinically mimicking a peroneal neuropathy. That said, I don’t recall any sciatic neuropathy in my career where the EMG showed abnormalities of solely the common peroneal nerve and absolutely nothing to suggest tibial nerve involvement (like: abnormal medial or lateral plantar sensory responses, H waves, tibial F waves, tibial motor NCS, needle EMG of tibial muscles) or denervation of the short head of biceps femoris. Usually, a small degree of tibial nerve pathology is present.