Your line about 'assuming' they're no longer in the bass... that's nice as wishful thinking goes, but unrealistic. Once a boring larva has found a food supply it doesn't just leave. It lays eggs. The various beetles whose larvae consume instruments and other wood supplies are born capable of laying viable eggs, they don't even need to mate. Depending on the variety they can live as individuals in an instrument for anything from a few months to a few decades, going dormant as necessary depending on environmental factors. Ridding an instrument of borers used to mean gassing or injected poisons, neither very effective. Now there are other methods which if applied correctly can be much more reliable. A well timed freeze-thaw-freeze-thaw cycle can trick their metabolisms such that on the second freeze they are destroyed. Careful heating with humidity control to avoid splitting the wood, maintaining (if memory serves) a temperature of about 57 degrees Celsius for a short period works. Replacing oxygen with nitrogen for a few hours (or was it days?) seems also to work.
To your specific case though; if you're seeing holes like that, there are living larvae slowly eating wood in that bass. They do not just go away.