"Briar," you say sharply. "Go back, tell Amy somebody was spying on us. I'm going after them." Not waiting to see what the fairy says, you turn to your dog. "Moblin, get the scent."
Your dog blinks and begins sniffing the prints in the sand. While he works, you cast a spell to reveal any lingering psychic impressions left by the spy. You don't get much - the aura wasn't very strong to begin with, it's faded rapidly since its source moved on, and the Hellmouth's interference is hardly helping - but you're able to determine that your eavesdropper was either human or mostly human, and left no obvious magical signature. Just looking at the tracks, you can tell that whoever made them was wearing sneakers.
Moblin woofs once. You take it to mean he's ready to track, and follow him at a fast walk. You're right on the trailing edge of the aura, and with your spell up, you can literally see it fading away, like morning mist. You'd like to move faster to keep up, but if you go any quicker, you're likely to draw unwanted attention, and then you'll lose the trail entirely and have questions to answer.
The tracks lead directly back to the crowd, the majority of whom are still watching the Clown, who has moved on to juggling a mix of rubber balls, bowling pins, an egg, and - good grief, is that a knife? You quickly shake off your shock and pull your attention away from the spectacle, but it's too late; in those brief seconds when you were distracted by the entertainer, the aura-trail gave up the ghost and passed beyond your ability to detect. Moblin is having similar problems; he snuffles around on the edge of the crowd for a minute, getting a few laughs and squealing protests from the kids, before turning back to you. The shamed, apologetic expression on his ugly face tells you clearer than words that he's lost the scent in the crowd. Attempting to follow the trail by mundane means is pointless; it's been obliterated by the stomping of a hundred-plus pairs of feet. A lot of those feet are bare or in sandals, but enough are still wearing sneakers to make the trail irrecoverable.
Foiled, you look around, hoping against hope that somebody will do something to give themselves away. Some of the nearest kids catch your dire expression and flinch, quickly looking away, but you don't sense real guilt in any of them, and no one else in the audience reacts.
One person does notice. The Clown catches your eye, winks once, and grins even wider - and less sanely - as your frown ratchets up to a glare, just for him.