I use that fact quite a bit when training my labrador; when in the woods, if she ranges too far, I'll just stop and stay completely motionless. She'll fail to see me at about 15-20 meters from me, especially if I'm against a tree or something. Even if I'm not obscured at all.

Soon tho, her nose will pick me and I can then see her eyes 'click' on me eventually. I use that trick to keep her "worried" she'll lose me, so she keeps checking on me at shorter intervals and control her urge to follow a yummy smell.

The interesting thing is that on the other hand, she has no problem tracking a moving object in flight at 100m, like a pigeon. Once I was shooting clay pigeons, and missed one overhead, and a few minutes later she came back all happy with my unbroken clay target, she had tracked it, marked it, and retreived it!