I think its the vent hole at the top of the res on the cap if anything - theres always some "ATF mist" around the area IIRC -

Some guys I remember had put a piece of a cotton ball in the "vent hole" in order to avoid that -

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2018 Ram 2500 6.7L Cummins 68RFE
19k miles -Bright White/Black - Big Horn Sport - Crew Cab Short Bed
2013 X5 35D (CEO's) - Born on 5/17/2013 -
82k miles - Alpine White/Cinnamon Brown/Premium Pkg, Sport Activity/Premium Pkg and Sound/20" Style 214/Running Boards