Giant snakes are hard to find even when you know they're near

From an article about invasive species:

Biologists who study snakes often discuss how difficult it is to find our subjects in the wild. Knowing this, Michael Dorcas and J.D. Willson (two snake biologists) recently tried to figure out just how effective people could be at finding pythons. To do so, they placed 10 full-grown Burmese pythons in a 102 by 82 foot semi-natural enclosure containing vegetation, a small pond, and trees. Then, after the pythons had settled in, they allowed 19 people to come in one at a time and look for pythons for 30 minutes each. Although each of the 19 people could have potentially found all 10 snakes (a total of 190 potential finds), only two finds were recorded over the entire study