I didn't mean to alarm you--you should always use caution when cleaning up rodent droppings, but generally Hanta virus cases happen when a large amount of droppings is being swept up in an enclosed space--like cleaning up an old garage or a nest in an abandoned mattress. I don't know how old the droppings would need to be before the Hanta virus wouldn't be a threat, but a quick google search would probably tell you.
I always spray droppings with the bleach solution before sweeping or vacuuming, but I don't think I'd be concerned about droppings behind the walls unless there was piles of it. If you're still hearing scratching, though, I would have a pest control company come out for sure!
We periodically have had mice in our wall (twice in 4 years, at least) and can't for the life of us figure out how they got in. We have looked everywhere to find the access point. In one case, I think the rodent was stuck as it sounded like it was repeatedly trying to jump, poor thing (either that or it was a bird and trying to fly) and after a few days I stopped hearing it.
I never smelled anything, but I do think it is still in the wall....
Just a few weeks ago, we had another one in the wall, scratching and gnawing all night long, and finally had to cut a hole in the wall and tape a box with a trap in it in front of the hole. Over 3 days we caught 2 mice. One appeared to be pregnant. We haven't had any since, but we keep trying to figure out where they got in. We know it was a problem before because there was a mouse trap under the gas fireplace next to the problem wall when we moved in.
The house is in the country, and we have A LOT of mice out there despite our barn cats' hunting prowess--so we expect to have the occasional mouse in the house. But having them in the wall, living and multiplying is another thing, and if we hadn't been able to trap them we would have had a pest control company out quickly. They reproduce quickly and in large numbers. Problem is, the poisons they use will also poison our cats if ingested when the dying mice find their way outside (my understanding is that the poison makes them severely thirsty and they go out to find water, and die outside, ideally).
I don't think there's a way to clean out the walls or under floorboards without pulling them up/out, and I'm not sure it's worth the effort. The main thing is to plug up access points and kill/remove the rodents currently in the house.
Good luck--it's gross and nervewracking but not the end of the world. The thing with mice is that one little guy can make so much noise and mess! Ugh. And as someone noted above, if you really think there has been a LOT of activity, I'd be concerned about wiring as well.
I wouldn't sell a house because of prior rodent activity within the range of normal.
Please register to participate in our discussions with 2 million other members - it's free and quick! Some forums can only be seen by registered members. After you create your account, you'll be able to customize options and access all our 15,000 new posts/day with fewer ads.
Mice Infestation
It's one of the truths of old houses they don't really say too much about on the HGTV rehab shows. MOST homes have or get mice in them from time to time. 
