We monitor our network for excessive new outbound connections(>1000) in a 5min rolling window. And this must repeat more than 10 times in and hour.
This is the hallmark signature of malware somewhere on your local network.
Be sure all your devices have their security software up to date.
You can also run tools like Malware Bytes that help alot. There is a free version of that particular tool.
Here is a site that provides some useful information on this topic.