Green’s Dictionary of Slang

1. stains of bodily fluids on sheets or underwear.

1985		‘Thommo’ Dict. Aus. Swearing & Sex Sayings 121: SNAIL TRAIL — The remainder of male or female love juice which has dried and turned hard on the blankets or sheets after masturbation or sexual intercourse.
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.