"You are looking for your scarf."
Translation:Tu cherches ton écharpe.
Exactly, "ton" is used for euphony. "But echarpe is still a feminine word." The rule applies to feminine words... it doesn't apply to masculine ones because masculine already uses "ton"... so if someone tells you that ton is used for euphony, then if it didn't "override" the fact that "echarpe" is feminine, what would be the point?
In short, it doesn't matter if the word is feminine or masculine. If it starts with a vowel sound, then you use "ton" for informal. Period.