its hard to say what it is without being there. but it could have been any kind of predator. Is there a barrier there like a fence, road, stream, even a change in the crops such as high weeds to cut fields? Was the wind going to where they hung up? is this the first time this has happened in this field? if it isn't do they come in the same way each time? Did you change the sound at all on your spitfire after they hung up or did you stick to one sound?
without knowing your situation I would have tried a squeaker and if they still didn't commit I would wait till they left and then leave myself. if it would happen again like that I would setup closer to the edge with the call in the same place you had it before and call. thinking on it a bit it sounds like fox that have coyotes in the area. or you have overcalled the area. JMO. you made the right call not shooting. reccomendations to a scope are numerous. I have a bright enough light that I can id animals at the ranges I hunt in. 100-150yds.