Some of it is concerning, some there is most likely a good excuse for. The non opening of the windows, I LOVE my windows open but I can't ! because of cats, wasps, flies etc so keep them closed, I would hate to think the neighbours are discussing this, although I doubt many notice. There could be a reason like this ?
The baby never leaving the house . Are you sure ? is it at all possible the baby is brought out at a time you aren't watching her house ? Could the baby have an illness ? my youngest was born severly disabled, she had no immune system and I couldn't bring her out for the 1st year of her life, could it be something similar ?
You say when you are looking in her windows you can't see her holding the baby, could she be holding the baby in the other side if the house ? if my neighbours at the front were watching me they would think I didn't have children, that's because we live most of the time in the back of the house. Could the baby be napping when you are watching her through her windows ? she may have a travel cot or moses basket and the baby in that, none of mine had to be held 24/7.
All children develope differently, some speak earlier than others, some not at all, could it be possible the child has SN and she is getting them help and you don't see her bringing him to appointments ?
The wandering is awful, although alot of parents allow this, I have read threads on here of children as young as 2 years out un supervised.
The mother had the baby 9 months ago, could she have PND ? maybe she can't bring herself to leave the house alot, maybe it's alot to do the small bit she is doing ?
If it were me, I'd get together with the other neighbours who are watching her and come up with some sort of arrangment, something like dropping off a food dish, or a cake so as to get talking to her and see if she looks like she needs some help. She could be perfectly fine, and you could be missing certain times she does go out, does do things, I remember my eldest only slept during the day, at night I would walk him around the estate, I'm sure neighbours were wondering where I was during the day as I napped too when he did and then they'd see me heading out at 2am with the pram, he had bad colic and the only relief he got was at night out for a walk in his pram.