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I too would suspect it was a breather tube. Some of them are made of fairly hard plastic and can be devil to remove. You could try tracing where it goes. In the past I have had to use hot water to get the pipe to disconnect easily but don't use a naked flame as the breather tube is designed to take away any hydrogen gas that builds up inside the battery. Follow its route and see where it goes to.
Currently in madnoel10's garage:
Honda Civic 1.4l