This actually happened a few days ago.
I watched this one long chemtrail flaring brightly in the sun; the latest and brightest among a dozen or so visible trails. The sky was also full of very thin translucent clouds, the residue of earlier spraying. This bright trail traversed a gossamer aerosol cloud. It seemed to be casting a shadow onto this veil. As I was wondering how this could be possible I saw that what looked like a parallel shadow was actually a negative trail of a plane that had flown an almost parallel route ahead of the bright flaring plane. What I was seeing was a path cut by a plane at approximately the same altitude as the bright plane but had left no trail. Instead it left a negative trail as it cut through the nebulous cloud. In addition I saw other planes at approximately the same altitude (about 40,000 feet) leaving no trail. 4:30 PM. (Of course this means that the planes leaving trails were spraying, since the atmospheric conditions are the same for them all. If one plane above 40,000 feet is leaving a trail, they all should be leaving trails. Obviously.)