A fox. Do you have those? They can be outside and the scent can waft in with no problem. I had this issue in a place I lived in before, thought for sure it was a skunk, turned out to be little foxes running about. The scent's probably just coming through the air vents or an open window. I highly doubt a snake would bother to climb the stairs or that it would smell that strong. Some do smell, but in my experience you pretty much have to handle the snake before you notice the smell. The scent won't just fill up a room in my experience.
If there are homes nearby, sometimes people who cook meth use chemicals that can resemble the smell of skunk. Next time you smell it walk a circle outside your house and see if it's the wind carrying it in your direction. If it's not drifting through the air, but is localized to somewhere right outside your house, it's probably just an outside critter wandering about.
Ladybugs do have an odor but I've never equated it with skunk. We had a major ladybug invasion where we lived one year and the news was giving advice on vaccuuming them and such. We had a lot, but the smell was pretty minor. Well, except when a really big one flew into a lamp and fried to a ladybug crisp. EW It was our friend...we got rid of all of them but that one because it was really pretty, then that happened. The kids weren't too happy...
I hold you close with my right hand, hold Me, do not falter For long before you knew was need, there was Living Water