Looking at your pics, the wetness is more concentrated towards the engine, not the tank so I don't think it is leaking from the tank side. If I have to guess, it is leaking under the rubber of the clamp in your pics. Moisture gets trapped in there and caused a pin hole.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked