Could be the way sound travels. Bedrooms and their balconies of both houses face the back of the houses and I thought the bathroom of OP's faced the front of the house, doesn't it? If so then since Mrs N never left her bedroom I could understand her not hearing them...
I'd never have believed it until it happened to me. I live downtown on a block of brick houses, each about 6' from the other. I don't like AC; love the love the sounds of birds and smell of freshly mowed grass too much.
About 3 years ago, I began to hear what I thought was a soft beeping noise, about every 45 seconds. Sometimes I could hear it outside, sometimes I couldn't. Then it would come back. It drove me NUTS!!!
I stood in the front of my house, didn't hear it day or night. Nor in the back of my house. Just in my room, which faces the house next door ( a sidewalk runs between them.)
As time went by I got more and more agitated. It was like water dripping, non-stop, in the same place on my skin. Drip drip drip drip drip. When fall came, I shut my windows and I couldn't hear it. But when I opened the windows the next spring, there it was AGAIN!! ( Flashback: I had been dragging my sons down stairs, from the very beginning, asking them if they heard it - unlike OP who never checks in with anyone!)
I was really crazed by now, literally on the edge of madness. I went out to the alley behind the houses, up and down the block and back the other way to the end of it - ALWAYS still hearing that sounds... sometimes. How could I NOT find it?? Finally, I started checking other block and wouldn't you know, I FINALLY found that sucker -- someone's battery had gone low on their smoke detector on their back porch, began it's warning beeps and they just let it beep and beep. They never changed the battery out.
How it beeped for 12 months, I'll never know. But I knocked on that door, seeing red rats, and tried to stay calm as I asked the women ( plural) who lived there whether they could put a new battery in - PLEASE. AND, why hadn't they heard it? Why had it not bothered them?? They just stood there looking clueless about the whole thing.
I was glad I found it and it STOPPED but I was INCREDULOUS that no one but me heard that darn sound ALL that time.
Admittedly, however, I AM the one who mumbles. I probably have "dog ears," too.
But THAT's when I learned just how clever an adversary that sound can be!!