I am about 1/2 of the way into my phase 1 flight testing. I routinely inspected the engine compartment for anything that could cause problems. After doing about 5 hours of flight testing, I thought I smell fuel. I tracked down the tanks and found nothing. Today, I removed the upper cowl and discovered the fuel had leaked through the fuel sensor at the firewall. I had thought I torque the fuel sensor but I didn't torque it enough. It took almost 20 hours for the problem to reveal itself. Chalk it up for luck that I found it. This is the picture with the blue fuel stain.