When taking students outside, I always recommend to new teachers and interns that they purposefully scan the recess area with their eyes and be aware of any sounds. If something makes you uncomfortable, assume that it might be a danger and follow the school's procedures such as calling the office on your cell phone or taking the students back inside. Often the danger is benign, but better safe than sorry. Dangers I've encountered over many years of teaching: an animal control officer with a rifle who was quite upset that the school did not get the message to keep the students inside due to a rabid dog; a hunter who told the responding police officer he "didn't realize" he was too close to the school (duh!); a possible copperhead (it wasn't, but the teacher had to identify it first to be safe); broken glass; a worker spraying weed killer; a large tree branch snagged and ready to fall; etc.