I know I have heard of this happening before, but I can't find it.

MYSTERIOUS stringy filaments seen falling from the sky have mystified residents and authorities in the Drôme and Ardèche.

Reports of the fibres were first raised on Thursday, but no one has yet been able to identify what they are – giving rise to theories that they are the by-product of plane exhausts.

Some website are already speculating that the fibres are proof behind a conspiracy theory that "chemtrail", contrails from planes that take a long time to disperse, are deliberately injecting chemicals into the atmosphere to spread disease and kill the sick and infirm.

Others have offered suggestions from spiders’ webs to 'Angel hair', believed by followers of Raëlism to be made by UFOs passing through the atmosphere.

The prefecture for the Drôme said it did not know what the fibres were but said it was not worried and would not be sending them for analysis.

The Commission de Recherche et d’Information Indépendantes sur la Radioactivité has said there is no concern about radioactivity from the fibres.
Mysterious fibres fall from the sky in the Drôme and Ardèche – The Connexion