Leaking heater-core? Anyone?

Some time ago I posted a topic about coolant loss and the sweet smell of it inside the car.
The ones that replied were both convinced of the leaking heater o-rings being the cause despite the fact that there was no trace of water in the footwell or anywhere else.

So I removed the glovebox and with a small mirror and a torch you can very well see the spot where the o-rings sit but no evidence of any residue there. I placed some paper towel under the connection point of the 2 pipes going in and out of the heater, just where the temp sensor is wrapped around it. If there was any leak, it would certainly show on the towels.

After a day of short distance driving with multiple cool downs and warm-ups (when the smell is most noticable) I lost about 200cc of coolant and when removing the fillercap on the reservoir there was a hissing noise. So some pressure was in the system. While doing so the level did rise. But the paper towels were still dry, no trace of a leak there.

Apart from the smell I cannot find where the coolant goes. Has anyone got experience with a leaking heater core and does anyone know where the coolant leaks on or into if the leak would be in the core? Or any other suggestions are greatly apreciated.

Regards, Olav