Why would holes exist behind a baseboard?
I know this is an old post but maybe my response can help someone in the future. We had a water leak in our bathroom that saturated a few baseboards. A water mitigation team came out, removed the baseboards and drooled holes exactly like this around the perimeter. They have set up multiple fans and dehumidifiers to dry everything up and will then put a mold treatment down to prevent any mold from forming. I was told new baseboards  can just be put up. So, looks like a past leak and hopefully it was treated properly before putting those boards back up.