We had a similar problem when our wood frame place was new. Bats don't need a big place to squeeze in.
We caulked every crevice and the problem stayed outdoors. My dad inadvertantly tarred-in a bat, and it was left to me (being the budding zoölogist) to pull him out of the (real) tar glob. I spruced him up with a mineral spirits sponge-bath, and when released, he flew off like nothing had happened.
1) I don't think you have an "infestation" -- per se. They find a cozy "niche" and, come nightfall, forget which way was "out".
I think our main trouble spot was a vertical crevice -- and located high -- near the peak. Seal every crevice.
You'll likely always have them around outdoors. Limiting night-lighting might help. Night-time outdoor lighting bring moths. Moths are a too-easy protein meal for bats to pass up. Spiders, too, will set up shop near a light for the same reason.
2) Set up one or more "bat houses" away from the house, and out of the sun. The wire gauze inside -- and optimal spacing within -- make bat-houses an ideal place to hang out during the day. (Away from the house).
3) New Hampshire bats are supposed to know to hibernate in Vermont caves, when the cold weather limits the bug supply. Some bats, like yours, don't always get the message. He was most likely in "hibernate" mode, as you suggest.
4) I still get bats in little cubbies in the underside of the cottage, but none have gotten inside. You may need to check underneath your place, where openings were cut for wiring and pipes to pass through. New Hampshire's newest houses aren't being made as well as they used to be, and likely have more crevices -- for more appliances -- than older homes.
It may be possible to use an infrared heat sensor and simultaneously heat the interior while fan-ducting air pressure inside. It may be possible to analyze exactly where the heat losses appear outside, but don't know if that's a sensitive-enough indicator for your relatively small losses.
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