Well, I went outside last night and spent a little while searching the trees for signs of bats with no luck. However, if I point the detector in one specific direction it does pick up a regular ticking type sound. I tried standing in different parts of the garden and even setting the unit on the floor and walking away in case it was my presence causing the problem but all I could detect was this one ticking sound from a very specific direction and elevation, so I’m sure it’s picking up something very specific.

So, for now I’m assuming that the detector is working and it’s just that the bats don’t want to show themselves.

I’ll try again tonight.