I had a leak there that looked the same. It turned out that the plastic fitting on the tank pipe was not tight. It allowed some seepage when the tank was hot, extra pressure maybe.
2011 351RLTS Eagle, Mor/Ryde suspension & pin box
06 F350 Lariat PSD, SRW, LB, CC, EGR delete kit, 16K DrawTite hitch, Timbrens, TST TPMS
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