Secret listening house on Bute

 There's a lone house in the distance down this road at Scalpsie Bay on Bute.

Here's what the interpretation board had to say about it.

This surveillance will be possible because each design of propellor will have slightly different types of movement and vibration. It is possible to identify the frequencies that make up a noisy complicated sound by the process of a Fourier transform to produce a spectrum showing the amplitudes of each component frequency. Each propellor will have its own distinctive spectrum. That's important because our own submarines go up and down past Bute too!