The battle to protect your computer and network against treats is essentially never ending. While there are a lot of simple and effective things you can do, another trick to add to your arsenal is keeping tabs on your network and the connected devices. A simple (and free) utility to help you with this is RogueScanner.
RogueScanner scours your network for connected machines as well as devices connected to those machines. You can then view the findings of the scan to make sure what was detected is indeed what you have. If you find any differences, you might have something going on.
There is a RogueScanner tour available for you to view in order to get a better understanding of the process. This is worth looking into if you have an entire network to keep track of.