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Air drawn into the suction side of the fuel system, between the tank and the pump, where the line is under vacuum rather than pressure. That is what makes it so hard to find: a joint under vacuum draws air in without letting fuel out, so there is nothing to see, no smell, and no wet patch to point at.

The symptoms are erratic in a characteristic way. Air compresses where fuel does not, so delivery becomes inconsistent, the high pressure pump cavitates, and rail pressure fluctuates under load while behaving at idle. A diesel that starts poorly after standing overnight but fine after a short rest has usually drained its lines back through the same leak.

Look at the joints that are disturbed and the ones that age. Filter housing seals and their bleed screws, quick-release couplings, the tank sender's O-ring, and any hose clamped rather than crimped. A clear section of line shows bubbles, which is the quickest confirmation available, and a pressurised leak-down test on the suction side will find what inspection cannot. Suspect it especially after a filter change, since that is when these joints are opened.