I’m a care giver for my elderly mother. I see here nearly every weekend. She knows when I’m coming, to within 30 minutes or so. And as she is often in her wheel chair, it would be stupid to have to have her let me in.
I just let myself in and announce myself.
With us also it’s not quite the same in both directions. Our grown kids do not knock when they enter this house, the one they grew up in. In a low crime US urban area, we’re not on edge about intruders, they say ‘hey it’s me’ as they come in and the safety part is fine. Two of them live within a few minutes walk so while they usually come announced or expected (for usual Sunday dinner), occasionally unannounced. The one who lives a couple of hours away doesn’t show up unannounced, but you don’t really ‘drop by’ at that distance.
I OTOH would knock at their apartments if I went there not sure they were home. Although that rarely happens. I go to one son’s place to walk his dog all the time, because he’s asked me to because he isn’t home: I don’t knock for the dog. 
There’s no rigorous logical reason I tend to feel like calling ahead or knocking but don’t mind that they don’t always do that, but it’s fine with me.
When I was their age, late 20’s-mid 30’s, I lived (same city as now) 40 minutes from my parents’ suburban home. I wouldn’t have gone to their house unannounced or entered without knocking if somehow I couldn’t get in touch prior to arriving. But besides the distance, I just wasn’t as close to my parents as our kids are to us.
I have my own key and I let myself in.
I usually contact my family to let them know I’ll be coming by on a particular day, though I don’t know when (depends on bus/metro schedules.)
I’m generally visiting for an event, or simply arranged in advance, so I just go in and announce that I’m there.
At my folks old house, where I grew up, I’d just go in, arranged visit or not. On the plus side to that house, the entrance is nowhere near the private areas of the home, you have to walk up stairs and go around a bend to even see a bedroom door.
Another: no knocking, but also no showing up unannounced. I suppose if I were to show up unannounced I’d knock.
My mom lives in a very rural area (my parents retired from farm country to…farm country). Friends and family know to never go to the front door, always to the back door where the kitchen is. The back door is only locked when no one is home. Only strangers come to the front door, which is always locked.
Friends knock and then walk right in.
Family just walks right in.
It took me a second to understand the premise of the thread, because visiting my parents involves flying halfway across the country. So when I arrive at their house it’s because they’ve picked me up at the airport. And of course knocking wouldn’t be required in that situation. It took me a while to realize the OP was directed at people who live close enough to their parents to just drop by.
I don’t knock and I don’t call ahead; however they virtually always are expecting my arrival anyway, either because I’m going to a prearranged event or they’ve specifically informed me that I have a package waiting there for me to pick up.
It’s a pretty big house and it would be inconvenient for them to have to come upstairs to let me in, particularly if I’m just fetching a package.
I consider my parents’ house to be my home, even though I live hundreds of miles away. So, if I’m in town and coming and going, I don’t knock.
My mother is getting a bit paranoid, due to her deafness and loss of sight. She doesn’t know what is going on, so she worries about it.
Consequently, I always ring the door bell. If she hears it, it gives her a chance to open the door, rather than finding “someone” unexpectedly in her house.
When my mother was younger, no. But perhaps my sister did. She took Dad’s key to her door away because she didn’t want him casually wandering into her house.
I don’t knock at my mother’s house- but that is at least in part because it’s a two-family house so I’m walking into a hallway, not into her living room. It’s also in part because that house has been entirely occupied by family for my entire life and it was never the custom to knock on the apartment doors. It’s possibly also because I grew up with relatives just randomly dropping by to visit my grandparents. ( If they weren’t home the relatives would drive a few blocks with the pastries to another relative’s house. Eventually they’d find someone home). I usually do call to her as I come in, because I don’t know exactly where in the house she is. My kids just walk into my house - my daughter lives too far away to just drop in without knowing if I’m home so I know she’s coming, but my son lives closer and does come by without checking to see if I’m home ( because he’s coming to do laundry and I don’t need to be home for that.)
I knock pro forma but go in without waiting for an answer and announce myself in the foyer. But I also don’t just show up unexpectedly without letting them know I’m on my way.
Knocking is redundant. If Mom’s home when I get there, then the dog will bark his fool head off so loudly that she couldn’t hear the knocking anyway. If she’s not, then the dog will be quiet, but there’s nobody to hear the knocking, either.
I’ve lived in a few different countries, but never lived anywhere where the front door was openable from the outside without a key, or there was trivial access to the rear of the property. So this thread is an insight into american living for me!
But anyway, answering the question, if I have brought a spare key to my parents place, and they know I’m coming that day, I’d just let myself in. If they don’t know I’m coming that day, I’d knock / ring the doorbell.
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When I stay at a relative’s house, I would knock the first time but then let myself in through the rest of the stay. A parent’s house was just a special case of this where it was really unlikely that knocking the first time was needed since they were right there with me.
I almost never knock at my parents’ house. But they almost always know when I’m coming. We often text about plans, so if I say “I’ll be there around five” and show up around five, I’m just going to walk in the front door.
If for whatever reason I show up without their knowledge, I’d at least knock loudly and wait a minute before walking in to give anyone a chance to get decent if necessary. But they also have a long driveway and a dog, so nine times out of ten, they see someone coming and have opened the door before I even get out of the car.
Even after moving out, the place still feels like my home, though I remember being shocked at my realization a few years ago that I have lived at my current house longer than I ever did anywhere else, including my parents’ home. Well, we’re moving again soon, so I’m hoping to set a new record eventually.
I have a key to my parents’ house. But as they are on another continent, I never just drop by.
My MIL used to drop in on my SIL. My SIL took away her key.
This, exactly.
Our daughter, mid 40s, still comes in without knocking. That’s fine by me. I’m pretty much OK with having her witness whatever Ms Napier witnesses in the privacy department, and we’re not going to be undressed except in bathrooms and bedrooms which she would not enter without knocking and being invited. She’s inside the “house privacy zone”, you might say.
In my youth it was different. I moved out at 16 and would walk in without knocking if I visited the house. But when I was 19 my mother moved away, and I never had a key to her home.
We are talking about our parents. Would it be likely that one would be unaware whether one’s own parents were likely to walk around the house other loaded weapons?
And why “these days”? Last I read the percentage of people who own firearms is shrinking.