"...Could an EMT, medic, nurse etc even tell it happened..."

I have seen a couple of severed arteries, but never a femoral, so I am just guessing here. One would be able to tell that the artery was cut from the volume and type of blood. As far as it retracting, I just don't know. Not much an EMT, etc could do about it anyway, other than pack on dressings and pressure...