I lived alone for about 7 years. If I had anyone coming for whatever reason, I would always be properly dressed and tidy. If I wasn't expecting anyone and someone knocked, I just wouldn't answer the door.
As for my knickers - I generally used to put them on the washing line in the garden. On wet days I would hang them in the day room. I did get caught out once with that though. I was working from home one afternoon and was expecting a delivery. Knock on the door and it was an old friend of my parents. I naturally invited him in and made him tea. It was only after he left that I realised all my lacy smalls were draped over the furniture. I was mortified!