Some missions let you explore more areas than others but sometimes certain areas are unlocked. If you really don't want to get called back, just don't finish the last part and go exploring everything you can.
I'm in the third level/area right now, I haven't seen anything that lets you free roam but I really don't see the point.
I'm in the third level/area right now, I haven't seen anything that lets you free roam but I really don't see the point.