It could be a leak due to excessive copper loss due to a chemical imbalance or sometimes even due to high or turbulent flow. Or a combination.

Some pinholes can be easily DIY repaired, some can't. If you look at a parts diagram for your heater you can figure out where exactly the leak is...and get a part number for replacement if that's the route you need to go.

The solution? Test your water. If needed, balance it.

If your water is fine consider how you add chemicals to the water. If you have any sort of injection system the injection site should be in the plumbing down-flow of, or after your pool equipment, so you don't get chemically saturated water flowing through your equipment.

Be careful how you add chemicals, example if you use chlorine tabs, they shouldn't go in the skimmer box. When the pump is off the tabs can result in very high levels of chlorine that can work their way towards the copper when the pump finally kicks on.

Thanks so much for your input - much appreciated. We will definitely be checking the water to ensure this doesn't happen again.