This story is another one that's more my Mum's experience than mine.
My Mum has been a stay-at-home housewife since quitting her first and only job when I was born. At first she coped well with this, but around the time I turned 18 she was forced to give up her driving license (due to a huge decline in her eyesight). She could no longer travel to visit friends or spend much time out of the house. As a consequence, she always used to get very lonely on weekdays whilst my brother and I were at college and my Dad was working.
At that point in time her sight was so poor that she couldn't even read or watch TV for long before her vision completely faded and she'd need to stop to rest her eyes. She used to give herself extra unnecessary housework to do just to keep herself occupied, and she was always very down and unhappy.
She began to hear footsteps and doors opening and closing around the house whenever she was alone, but when she looked she could see nobody there. She tried closing all the windows in the house in case it was caused by the wind, but the noises didn't stop. They became more and more common until one day she heard the footsteps stop behind her and felt someone blow into her ear. Then she started hearing little whispers that she could never quite catch. She was very unnerved by it and she would often ask my Dad to drive her to town on his way to work and pick her up when he finished so that she wouldn't have to be alone in the house all day.
However, a month or so later when my Dad didn't receive the pay raise he'd expected due to government cuts, he could no longer afford the extra fuel for driving her to town. Stuck in the house again, Mum decided that she would just try to make the most of the company and not let it freak her out. She started to talk to her "invisible friend" whilst doing her housework, until it no longer scared her. Sometimes whilst she was talking she would glimpse a shadowy figure from the corner of her eye, just standing there watching her. It never responded to anything she said or did, but she felt happier and much less lonely with it around.
She didn't tell my brother and me about it, but she spoke to my Dad a few times. He was convinced that she was just imagining it all or her poor eyesight was making her see things that weren't there, to the point he suggested she see a doctor about it. She visited her GP who, sure enough, put it all down to her poor eyesight and depression. She was referred to a vision specialist, who managed to operate on her eyes and gave her some newly-developed glasses that were much stronger than anything they'd been able to prescribe her before. This restored some of her sight - enough that she could read, watch TV and use a computer properly again.
For a while she didn't hear anything more from her "invisible friend." But one day when we were together in the kitchen waiting for Mum to finish cooking the dinner, we heard footsteps crossing the landing above us. My brother and I ran upstairs to see if anyone had gotten into the house, but found nobody and no other explanation. Our neighbour wasn't home so we couldn't even pass it off as him walking on his side of the building (though the sounds would have come from a different place then). We were a little jumpy by this point, and went back to the kitchen as fast as we could. When she saw how freaked out we were, Mum confessed the story to us about her "invisible friend" and said she didn't think it was anything to be worried about.
Now we all hear the footsteps on occasion, although a lot less often than Mum used to, and mostly have gotten used to it. (Though I still always have to check around the house in case somebody has managed to break in every time I hear anything.)
We're not sure what it is or whether it's actually nothing and there's some way of debunking it that we've missed, but it certainly helped my Mum out of a rough patch to feel she had something to talk to.