I have seen bicycle chains that have some sort of covering on it?

Its made of some type material or sleeve. It covers most of the chain and is black and sleeve like but narrow so just goes over the chain. I was examining one the other day on a bike, and the owner came back and told me to leave his bike alone. I tried to say I was only checking out his chain but he was rude and... show more Its made of some type material or sleeve. It covers most of the chain and is black and sleeve like but narrow so just goes over the chain. I was examining one the other day on a bike, and the owner came back and told me to leave his bike alone. I tried to say I was only checking out his chain but he was rude and told me to F off. Rude. I couldn't ask him about this device and what it does. It doesn't look water proof and sure I can cut with pliers, so why is it there? and what is it called?
Update: sorry - It was around the padlock chain, holding the bike to a post, not the chain on the bike. I could have been clearer apologies
Update 2: ITs NOT a chain guard, its around the security chain NOT the chain on the bike!! Please read the update before answering
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