Is a lunch interview a good sign a manager is interested? ?

No, it's just a sign that he wants you to pay for his food.

Not always.

I remember when I was looking for work I contacted a guy I used to work with who helped hire for a small consulting company.  He said "Let's talk over lunch.  Join me at my favorite restaurant."  I went there and we had an enjoyable lunch with him praising the place, which was fairly expensive. Much of the conversation was talking about family, people we used to work with, etc.  At the end of the meal, I realized that he really wasn't looking to hire me but needed an excuse to put the meal on his expense account.  So, we both got a free meal paid for by his company, but that was the end of it--I didn't hear from him again.

Didn't you ask this yesterday?

You keep asking the same question and getting the same answer.  Meeting for lunch is NOT a good sign.  It means that:

1. The office is closed due to Covid

2. The opening/hiring is going to cause some kind of trouble/ruffled feathers so the manager is hiding the interview from other staff.

3. The manager hasn't actually gotten approval to move forward.

4. The manager wants to see how you interact in a social setting.

You should ASK why he doesn't want to meet in the office.  Even if, as a sales person, you might work from home or not in the office, it is rare to have a first interview off-site -- Medtronics or not. 

Not necessarily. The interviewer just might be hungry.

Neither a good or bad sign, it is a sign he has available time to see you thats all