Additional question - Can you give further details on the observation e.g. behavior, habitat etc:
Habitat: A lot of forest around here. I got the feeling they moved around between the trees in my garden and flew around in circles.
Behavior: I could hear a few bats flying around as mentioned above in circles. And why I'm making the assumption is because I could hear the sounds from them getting distant (like 10-15m away) and getting closer and so on.
Observation: I couldnt see one fully but a dark "lump" flying around is all I saw. On the other hand one flew so close to my head I could feel the wind from it's wings.