Shadows

There's a saying that crocodiles can grab you by your shadow and pull you into the water.

The saying definitely has some truth to it. If you're close enough to the water, that your shadow can touch it, you're close enough to be grabbed by a crocodile. When the sun starts setting your shadow will naturally grow longer.

Crocodiles also tend to feed near dusk which corresponds to more aggression and a larger danger zone. Thus the longer shadow will also represent more danger.

Of course, if you're extremely smart, you'll avoid any waters that might contain crocodiles or other dangerous animals.