The only condition I have ever heard of is the light's tail-off showing up in a highly reflective surface (such as a mirror). The flash would have already fired and be cooling down, but you might see a faint glow.
You can see more than just the tail-off in a mirror. You can see the actual flash. It is easy to test in a mirror yourself., set your slave off to the side and have your camera and masterflash pointed at a mirror.
Nikon actually makes a Nikon SG-3IR IR Panel for Built-In Flash to surpress that when using their built-in flash as a master flash controller.
I don't think it would be compatible with Canon gear (without some Dremel work) as it would probably press the button on the hot shoe to indicate there is an external flash there by disabling the built-in flash.