If the OM606 is anything like the OM603, then there is a banjo bolt on the back side of the injection pump (towards the engine block) for the return fuel line that can leak. Don't rule out the lift pump either. You have checked the O-ring seals on the top where the delivery valves are for wetness right? That is a VERY common place for fuel to leak on any OM60x engine.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

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1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]