Bats go quiet during fall mating season.
Using specialized infrared cameras and ultrasonic microphones to record bats flying through river corridors in northern California, the researchers discovered that bats use short, extremely quiet “micro-calls.” At times, no echolocation calls at all were detected, even from bats flying within 10 feet of the highly sensitive audio recorders.
https://news.wfu.edu/2018/05/02/bats-go-uiet-during-fall-mating-season/