Quest of the Golden Ape by Stephen Marlowe

B'ronth the Utalian left footprints in the snow.

Otherwise, B'ronth was invisible. But if a hidden observer watched the Utalian's slow progress across the ice fields of Nadia he would see where the ice was soft or where snow had fallen during the night into the gullies, the unexpected, mysterious appearance of footprints, a left staggered after a right, then another left, then a right again, then a left.