Brake line dirt? Yes, a dumb question.

[PowerboostedF150]

Hey all, completely dumb question here, and no real issue here other than a curiosity. I noticed one of three brake lines with caked on dirt on the outside, but not on the adjoining ones. Truck engine area is lightly cleaned with spray regularly (not deep cleaning, or component removal). History of work was transmission work (was told it was torn apart by dealer, but I suspect they just dropped the pan) last April, new plugs (done myself) last September. Otherwise nothing else done.
Any reason why one would be dirt covered and not the others? A leak maybe? No other sign of a leak. 117k km, ‘21 Job1 hybrid.

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[Aonarch]

My guess it is leaking, maybe at the fitting above stream.