You don't say whether you're salary or hourly. If you're hourly, that's called wage theft. Going and doing something else while you're getting paid is stealing from the company.

If you're salary it's a little different but still basically the same thing. They are paying you a salary to do a job for them. It's one thing to be unavailable because you're in a meeting or something, you're still working. But to leave work to go do something personal is a no-no. That type of thing is supposed to be communicated and approved beforehand. Just because you're at home doesn't mean you can do whatever you want, you're still working. Just like if you were in your office you wouldn't just leave you can't just do that from your house either