Bats in the Garden
A few years ago I bought Hodmandod Senior a bat detector. We sometimes go around the neighbourhood listening out for them - but last night they came to us (as they often do). They can be identified from the frequency of their clicks (there is an app for this, of course). Hodmandod Senior identified this as a Pipistrelle - the common sort.
A bat detector reveals a different world: not just bats, but the rustles of all sorts of other animals can be heard as they shift in their sleep.