Can put a garden hose in one of the hatches and fill the kayak with water? That would be one way to find any leaks. Once found, you could use a short hose to siphon the water out.

Thanks. Yea, I tried filling with water and couldn't find a leak.

And the kayak gets VERY heavy; even just half filled I couldn't tilt it to drain via the drain plug.

I'll keep trying - soapy water and filling sections. It's a slow leak but I just don't want it to get worse suddenly, especially if I'm in a bay rather than a lake.

Scupper holes act as load bearing points between the top and bottom. Without you sitting on it, it might not be flexing enough to open the crack.