Why is my internet data usage high when not home?

Or someone flashed your router with an OpenWRT package like they did mine and are using it in a botnet. Just kidding: I mean it's probably automatic updates or the streaming devices doing the phone home thing as mentioned before. Additionally, devices are constantly talking to one another and sending signals back and forth. Some might see it as spyware but I think most of the proprietary question / response protocols are simply a harmless big data gathering technique by ISPs. I understand the discomfort and have a lot of criticisms of big ISPs myself. IoT devices can also use a lot of data or if you're on a mesh and you are correct about the anti-virus application because I know my ESET Premium shows a lot of connections when I look at my traffic. My advice, if you are noticing upticks and it keeps bothering you, take a look at your router traffic. If you own the router and it's a personal router and NOT a company's, then by all means get Wireshark and look what's happening. That way you can be positive. And yes, switch to unlimited 5G Internet if it's available in your area. ¡Buena suerte!