Yeh, the heater matrix probably drained of all the coolant (the heater uses engine coolant to heat the air) so with your leak then that will have drained all the coolant out.
At the same time, I would check the heater matrix as these quite often blow a hole and is the normal reason for getting a slow, but steady leak. So that is the first place I would look if I cannot see an obvious leak.
Try filling the coolant up with the engine running, you won't get the pressurised system, but you should still see the leak. The heater matrix is usually just behind the engine fire wall, you can trace the coolant pipes back to there. There is hopefully a drain, if not, you might notice your carpets are a little soggy...
Keep us informed!