What you're seeing is not unusual. As our POWDERPOST BEETLE ARTICLE explains, an active infestation can  remain in a small area for a long time. In fact I've seen this happen where the activity is noted in the same area for 10-20 years so it can happen. And your fears are legit; it's entirely possible there is other activity you cannot see. But I wouldn't panic just yet.

Based on your message, it's unclear if you even have beetles. I've seen situations where there has been sawdust falling out of both floor joists and subflooring but it's not been due to insects. Are you actually finding adults and/or exit holes?

Second, it's unclear where exactly where you think this infestation is even located. If you read your words, you say the "active infestation is between the joist" which makes no sense. Generally, the construction one would expect to see looking up while standing on the floor of an unfinished basement would be floor joists with subflooring on top of the floor joists. That means the beetles will have to be in either the floor joists or the subflooring but not "between" them.

Lastly, the fact that you have an unfinished basement could be a big plus in case it turns out you do indeed have a problem. Since all your floor joists should be exposed, they'll be easy to treat. I suggest you start there. If you have carpeting or some other flooring above which can be pulled up to gain access to the subflooring, I suggest you lift this covering and treat both areas with BORACARE. This would surely take care of any insects that might be active in either location. In the process you should be able to better assess any damage already caused so that repairs could be initiated if warranted.

Here are direct links to the information and products listed above:

Powderpost Beetle Article:  http://www.powderpostbeetles.com/powderpost-beetle-control

Boracare:  http://www.bugspraycart.com/insecticide/liquid/boracare-gal