There is a drain hole from the valley so any fuel leaning around the intake will end up there. The fuel filter housing is a common place for leaks. You could have old hoses leaking. You could have the O rings for the heater and water sensor leaking. You could have the sensors themselves leaking. The injection pump also has some places that could leak fuel but those are not nearly as common on these as leaks in and around the filter assembly.
A vendor like Leroy diesel sells the O rings in a kit so that is an easy way to do it. The water sensor isn't that unusual but the heater one is bigger than what comes in most assortments.
If you are going in most of us replace all the hoses as well as the o rings and while we are there we also relocate it just to make it easier to work with in the future. Most people raise it up on threaded rod. My solution which I thought was easier and gives more space was to just bolt it under one of the upper plenum bolts. That raises it high enough that its easy to route the hoses and change filters and doesn't require any new hardware other than the hoses that you already plan on replacing.
98 Burb K25 4.10's 250K miles, new to me, pretty clean and stock.
95 Burb K25 3.73's CRASHED 250K miles, looks to have been rebuilt at least once, stock, bad rust, DE exhaust w/walker muffler, 8' western plow.
81 Buick Riviera GAVE AWAY with the infamous Olds 5.7 diesel possibly still going strong.
15 VW Golf wagon, TDI. Did the buyback on my 13 Jetta TDI wagon so it was basically a trade up.
96 Burb K25 3.42's SOLD at 340K miles, told it was all original, close race between engine wearing out or body completely rust out.