A finder is a weave, a little like the Warder bond, though much less complex, laid on an object, that allows the Aes Sedai who created it to find the person who has it. On clothes, it fades in weeks to months, but is permanent in metal. Elayne used it on Samwil Hark, so that she could track him if he tried to escape (KoD, Ch. 16). It is likely that Moiraine laid a finder on the coins she gave Mat, Rand and Perrin, which enabled her to find them so long as they still had the coin (TEotW, Ch. 28).