I was at my girlfriends place and she had me all juiced up, raging hard on. Then I had to go. At home I noticed inside my underwear was leakage from said hard on. Resembling tracks. Sad Pecker tracks.
The original (1930's U.S.) version was simply urine droplets or splatter marks from a man's pants deposited while urinating.
Example: Peeing into a urinal is the hardest to avoid splash-back and getting pecker tracks on your pants. Further Clarification: A: There's only one thing worse than pecker tracks on your zipper. Z: Oh, yeah, like what? A: Zipper tracks on your pecker. Example 2: some guys notice they've got pecker tracks and purposely splash a little water on and claim that the hand sink faucet "kinda exploded."