Does the enemy see Hanzo's sonar?
Based on personal experience, when playing against a Hanzo you can see them fire a sonic arrow and see the aura emitted from it when it initially lands, however the aura is not persistent, and the sonic arrows don't look much different than his other arrows nor do they stay for very long. So looking for the sonic arrow on the ground doesn't really help you know if you are being tracked or not, and unless you see him shoot it, you probably aren't going to be able to tell where it is.