Odd for it to be flying in the day. It could be a youngster that's finding it hard to find food, or has something wrong with it. I think there was someone on here who found one out in the day last year. I think he notified the local bat group who were able to take it in, look after it and release it again. Worth a go if you see it out again. Can't remember who it was but a search should turn up the thread. I have a maternity roost in my roof but they're all gone by now. The females arrive mid to late May, but other species may differ. I think mine are common pipestrelles.