So … after filling the day tank with fuel (15 gallons), it apparently drained (siphoned) out. I installed a Lowrance/Navico Fluid Level Sensor to put the fuel level of the day tank (15 gallons) on the NEMA 2000 network via the sending unit. Once I was able to monitor the fuel level, I discovered the fuel was indeed being siphoned. After I examined the plumbing, I decided to switch the engine return (without siphon tube) and overflow hoses (with a siphon hose) to/from the day tank, and this fixed the problem.
