sudden coolant leak right rear wheel area, and now stopped

[Jinster]

I was idling my car today for about 30 minutes when working on something.

Suddenly the car started dumping coolant from around right rear wheel area. It was quite quick and sounded like a steady hose leak. Initially I actually thought it was the neighbour using the garden hose.

The leak then stopped after about losing 1L of coolant. No oil in the mix.

I then topped up the coolant, it required about 4L, so it was low on coolant already. Although I haven't noticed any obvious leak before.

I then ran the car down the street, sat it idling for a while, the problem hasn't occurred again.

This is strange because the speed of the leak when it happened, and the fact it just stopped leaking straight away. One would think if the leak comes from a broken hose it would constantly leak once the flood gate is open.

Does the Boxster have some sort of "over pressure" coolant dump mechanism? I theorize that if I was low on coolant already, the idling made the remaining coolant boil (it's mostly water, not much of the green stuff in there), and then some sort of pressure valve opened to dump the coolant? I didn't think this existed on the Boxster.

I replaced the water pump 3 years ago, so hopefully it's not that.

What could it be?