bats are common , they tend to use your house as a nursery till their young can fly (usual around 6 months) , dont disturb them as there if big fines for doing this . best thing to do is find how they are getting in and fill the holes . but make sure the bats are not using it and have left , once the holes are filled put up some bat houses for them to return to. if you just dont do anything they will come back year after year and the colony will get bigger every year. the droppings dont look big , but bats hang in the same place all the time and I have seen a ceiling come down with the weight after a number of years . The urine from a large number of bats tends to smell bad . hope this is of some help , best of luck ,
Reg