What is this?

[Nigel Whitwood]

I think it is to scare away rodents.

Yes. A search for K&K M5700N gives a description of the numerous lines of defence it provides to stop martins invading your vehicle, (or should that be Martians, in which case it's something different).

 

[Tamanddave]

Had a similar problem with something, probably a squirrel taking a fancy to my abs cables at the front, best part of £1000 to repair.
The van is on the drive by the garage door, in the garage one of these is running all the time, PestBye Advanced Rat Rodent Mouse Ultra Sonic Repeller Repellent - Whole House. Not had anymore problems since then in nearly a year, fingers crossed it stays that way!