I got caught out on an exercise once, fortunately, only by a farmer.
It was around March, and I was 'dug in' in a perfectly camouflaged hide, where even the wildlife didn't notice me. However, as the early morning temperature increased, condensation, rising from the 'trapped' heat in the hide, gave away my position to the farmer who, of course, knew every inch of his land !!
No sounds, no shine, no shadows, no silhouettes, no odd shapes or movement - just nature telling the truth !!