How did a bat get into my house?

My windows are shut tight with the airconditioning on. I have a gas fireplace. All my doors have been closed. I was home all afternoon and the house was quiet. After dinner I popped out into the backyard really quick and when I came back in there was a bat flying around my kitchen! i was able to get it out by catching it with a sheet. I am worried though, does this mean there are more?? Where could it have come in from? It's still daylight and bats generally aren't out at this time!