OP you know what you did was probably not a great idea on reflection, there were countless things that could have gone wrong in this situation, that don't even bear thinking about. Was the house locked or unlocked? If locked imagine if a fire had started in the room baby was in, and no one would have known she was there, or been able to get to her. If the house was unlocked, imagine if someone had came and taken the baby? What if that was the day baby, figured out how to undo the highchair restraints and fallen out?
BUT... Having said that, she was fine, nothing happened, and you know it wasn't a good idea. I know what it's like having a young infant, no sleep, and still trying to get stuff done, and I think most parents do things in that first year, that on reflection, weren't the best ideas at the time. I had a really colicky baby, who would only fall asleep if being driven around in the car. One day after no sleep the night before, and hours of screaming and crying, she finally falling asleep in the car. I was over the other side of town by the time she fell asleep, and I knew I needed a few basics from the shop, so I parked up outside the store, and quickly ran in to grab milk and bread. When I came out two women were stood by my car looking at dd, and seemed really concerned. DD was still fast asleep, so I quickly got in the car, and drove off. I was so worried they'd taken my number plate down, and we're going to report me. I think back now, and know it was a really stupid thing to do, but in my sleep deprived, desperate, stressed out state, it also just seemed like not a big deal at the time. I think really, I just needed two minutes in a shop without dragging out a buggy, and dealing with having a screaming baby in tow. I actually feel sick now, when I think back to what could have happened, and this is 9 years later. I remember telling my mum at the time, and she was horrified that I thought it was okay. To be fair, I think she then realised how badly I was coping, and she started helping look after DD for an afternoon once or twice a week, so I could use that time to just feel human again. Don't beat yourself up OP, and if the neighbour does report you, she'd have to have stronger evidence than that for it to go anywhere, so don't give her any reasons too.