Insects, bird nests.

Mice nibbling on wires maybe?

Mike

Facing the unprecedented and massive fleet storage worldwide which is needed in the current COVID-19 pandemic, Airbus has launched an active support to all operators about the grounding, parking and storage conditions. To complement this on-going effort, this article aims at reminding some of the key safety considerations for a proper parking and storage. 
— Read on safetyfirst.airbus.com/aircraft-parking-and-storage/