Copper sulfate corrosion in a head torch

I was amazed to find blue copper sulfate crystals in the battery compartment of this head torch. To be fair, the alkali cells were 5 years beyond their label date. The contacts are copper, but where did the sulfate ions come from? The were 2 AA alkaline cells in the compartment. The electrolyte is thus potassium hydroxide. Mrs B is suggesting that it might be from the plastic in the case. Rubber is vulcanised by using sulfur to provide cross-links. Sulfur trioxide will react with potassium hydroxide to make sulfate ions.