The side of the space I sit on is barely occupied. I hot desk with an officemate on opposite days, 3 days a week we WFH. When I step into the building, I have to wear a mask. When I walk through the empty side of the space, I have to wear a mask no matter if there are 0 or a million people in there. To get coffee and not walk with 50 feet of anyone, yes, mask.

There a bunch of developers in this space as well. They are huddled in a little cube farm. I'm sure their faces are 6 feet from each other, but the linger in the same space all day and share the same air. But if the get up and walk across the empty space, mask.

As for your situation, I'm sure there is no expectation to wear a mask, and while I'm sarcastic and snarky to my own detriment, I would not want to bring that up lest you give HR a good idea to monitor you.