Green’s Dictionary of Slang

1. stains of bodily fluids on sheets or underwear.

1999		Roger’s Profanisaurus in Viz 98 Oct. 25: snail trail n. Smoo (qv) juice in a lass’s well-worn scuns (qv).
2012		(con. 1943) G.S. Manson Irish Fandango [ebook] No snail trails [...] there’d only been one bod on that mattress recently.
2023		R. Milward Man-Eating Typewriter 284: A crusty snail-trail of semen down her thighs.

2. see happy trail under happy adj.