Still trying to pin point an annoying small water leak (1 litre per week) - the leak itself is not visible but the plastic under tray and top of the sump casting is wet as seen from the front of the car looking down

The car is only leaks while the engine is hot/pressurised - the water drips off the plastic under tray onto the floor depending on the chamber of the surface as to which side.
Have tried to understand where the water leaking at the front of the engine would come from - OK water pump seems obvious but it was replaced about a year ago so see no reason why that would be the cause. Would a leak from the thermostat housing/coolant bridge or oil cooler cover plate show itself on the under tray at the front of the engine?
I think I will be making a coolant pressurisation cap and do a cold pressure test - I would just like to try and narrow down the possibilities/source of the leak if possible.
