Commonly, caterpillars of the Elephant Hawk-moth (Deilephila elpenor) are spotted when they’re on their way to find a suitable place to pupate and hibernate. That is at their final instar, when the caterpillars are large and brown. Perhaps some 5% of the full-sized caterpillars remain the green color, which also makes it more difficult to spot them.
Younger caterpillars do a much better job hiding themselves. Since the species is common, it’s worth though taking a closer look at willowherbs, the food plant of the caterpillar. If the time is right the chances are good to find some. Below is a series of photos from summer 2016. I could spot a couple mid-sized caterpillars.