Considering its only when the pump is on, a diagram on paper would be where I started AFTER I pressurized the water system with pressurized city water. Then, while giving the city water time to work its way through any leak, I would study that diagram to try to locate where the water pressure from the pump enters the system and WHY the water pressure from pressurized city water would not be at that same point. In all actuality, I cannot imagine where that might be unless there is a short piece if pipe between the pump and a check valve that has been placed in the system to prevent city water pressure from reaching the pump. That would appear to be the only viable point that water pressure from either source would NOT be.
The leaked water could actually have just 'chosen' that exit point as it probably is the lowest point of your belly pan.
One last thought, if it is only when the pump is running, the amount of wzter pictured should resukt in the pump running constantly, unable to build sufficient pressure to cause the pressure switch to turn it off.
Good hunting to you-I spent two weeks locating a leak in our 2013 Classic that took 9 years to become visible, but when it finally did, could not be ignored. I attached a pressurized water hose to the unit, then stripped the entirity of the rear bedroom. 7 hours later, a very small water drop eased out from under the vinyl flooring at the bathroom wall into the bedroom. AFTER THAT, it took the eliminating of 6 possible PEX rings that had water on them that could have been the source of the leak before I noticed a hidden cold water line had a rust track on it. Removing portions of the bath vanity allowed me to access the screw on fitting to the sink valve. THERE WAS NO TAPERED GASKET IN THE PEX FITTING THERE! How long the very slow leak had been hapoening, I haven't a clue.
Two weeks later, I had it all repaired, unmatching flooring was installed as a source of the original was not found. An experience I never again want to repeat, but the point is, the leak was no where near where I expected to find it.