I mean, even if the building manager doesn't know anyone personally surely she'd go back and forth out of the building almost everyday to pick her son up from work so he would've seen a kid versus some hidden son plot twist like henna ie had
But yeah other than some super strict parent i don't think they'd care too much of a 20 year old run off unless he's causing trouble/took money to play around lol
unless she also hallucinated it, it'd also be worth it if only for the other residents sake to report that a man with a knife went into their building
Like Tsama pointed out, the twist I think is that Take is her son, and Yuuta is Take's son (so her grandson). She is totally disguising herself because for some reason her real identity is probably a criminal or something, so this is the only way she can care for her grandson.
We see flashback of Yuuta visiting the family grave with his father in chapter 4, which is how he knows how to write Konno in kanji, ergo we can strike Yuuta being kidnapped out of the possibilities.
Also the nursery girl pretty much tells us the answer where Yuuta goes off to. He was drawing the family gravestone, hence the tall rectangle and the gray crayon, but then he probably realises drawing a gravestone isn't really appropriate so he changed it to his apartment building. The gray spot being where their apartment room is located is purely coincidental, tho if not for that, no one will probably notice it.
Anyway, Yuuta probably misses his father (Take) and goes to the family grave on his own (like how unsupervised children wander on their own). This convinces me that Take is actually also the Ren from the first arc. Rereading the first arc again. Ren's son was born sometime in 2009, and his last blog post was in 2012, so 3 years had passed. Sasaki mentioned that it has been 1.5 years since 2012 meaning it's 2014, so Ren's son would be 4.5 years old right now, might as well 5. Assume some time has passed and he is nearing 6 years old (sometime in 2015). The nursery girl mentioned next year Yuuta will be old enough to enter primary school. Primary school in Japan begins for kids age 6, meaning the timeline does fit with Yuuta starting primary school sometime in April 2016.
Additionally, Yuuta's mom said it has been 3 years since Ren died. Assuming Ren died sometime in 2012 (around his last blog post), 3 years later would be 2015, again giving further credence to my deduction.
Edit: Yep, Chapter 4 shows that this arc takes place in April 2015, so Yuuta would be 6 come September and thus of age to go to primary school in 2016.