The play of shadow can sometimes create the illusion of seeing gaps in the clouds like hole punch clouds or distrails. These illusions may happen in thin cloud layers like Cirrus, Altostratus, Altocumulus and are usually the result of the shadow of contrails. For a correct conclusion a close inspection of the sky is needed and some points of attention are: shadow doesn't reveal how the sky looks above the cloud layer on which the shadow falls, and after a while the shadow gets usually  fainter when the contrails gets thinner. Ultimately the contrail can produce hole punch clouds when ice crystals start to fall into the cloud layer beneath, but then the sky will reveal what's above the hole.


The picture shows a layer of Altocumulus stratiformis translucidus perlucidius with a straight line of a shadow which was the result of a fresh contrail.