[confused]

I've had the cabin at the lake for several years. 2 yrs ago I get a bat inside in the middle of August. I figure it came in when a door was opened, no problem we got it out, everything is fine for the rest of the year.
Last summer bats twice, on one occasion 2 bats on the same day. We look over the entire house and can't see where they are coming in. Get them out and no more seen for the rest of the summer and fall.
This weekend I come up to the house. which is kept at 50 deg when we are not here, no one has been to the house in 2 weeks and low and behold I find a live, but lethargic bat laying in the sink. I thought they hibernated or migrated for the winter. I guess my questions are
1. Do I have a infestation of bats, is it a question of where there is one there are many?
2, How do I get rid of them once and for all? I am not happy sharing my home with them I want them out!!!!
3. Why would one show up in the middle of the winter? Was it hibernating or was it dying of starvation/thirst?
4. How do you find where they are getting in? I have gone around the house on the outside with a propane insect fogger to see if there are any leaks, I can't find a hole where they get in.
Any help, insight, words of encouragement are much appreciated.
Thanks