All of a sudden my 2.5 year old has noticed the security cameras at school and is afraid of them. When I ask him what is scary about them, he doesn't answer. He says it's there to keep him safe (which is what the teacher told him). He didn't nap in school today and going to bed tonight he kept staring at the smoke detector. How can I help him understand?
Perhaps the child isn't so much scared of the cameras, but they simply make him aware that a danger exists that he is now focusing on. Especially since you say that he's also been staring at the smoke detector.
By being aware that fire is a hazard (and having a reminder in the form of a smoke detector in his room) his mind might constantly wander towards the room perhaps catching fire. Likewise, I don't know what exactly the teacher told him, but if they gave any examples of dangers the cameras protect him from, he might now be thinking of those dangers every time he sees a camera.
It might help to reassure the child that he is totally safe and the risks of fire and whatever the school cameras are for, are basically zero and nothing bad is going to happen. He's way too young to be concerned about very rare dangers that adults take precautions for and also way too young to judge their likeliness correctly.
It's possible he's concerned about their function. The devises he sees others use (cell phones, flashlights, lights, etc.) react to prompts, but these are a mystery--perhaps they are beings of some sort? Although he's only 2 1/2, I suspect his concern is evidence he's capable of benefiting from a hands-on demonstration of how they work even if he's too young to fully appreciate their purpose.