The Bat Conservation Trust claims that they commute between their roosts and foraging habitats, although I don’t know why they would need to do that if food was plentiful near their roosts.
Many bats would roost in dead trees but as these are rarely left standing in managed woodlands they need to find other sites.
Most of they prey species are able to detect the ultra sonic pulses so the bats need to keep moving to find enough food.
When they come out of their roost they are generally warming up, having a look around and getting a feeling for the weather etc.
OP – If you have a garden that supports insects they may just come to hunt.
If they are around at dusk they they may be roosting near or at your house, you could spend some time working out when they first appear, and narrow it down from that.
For example, If the previous night was wet then they should leave their roost earlier as they will be hungry.