Only after posting this did I realise that I didn't exactly remember if Masaki's soul actually had been sent to Soul Society or if, instead, her soul had disappeared due to the Hollow's... influence, let's say... Could you... correct me if I'm wrong, please? Thanks!
When Masaki had died, her soul had gone to Soul Society. She wasn't entirely gone, but Isshin wasn't comforted by this. Living in spirit form still did not solve such things as shelter, clothing, food (if she would indeed need it). He worried for her sake, and that she would return to the cycle of life before the girls would go beyond the doors as well, and he could join them there (that he would find his wife when the time came was of no doubt to him)
And so, when Ichigo had left for Sereitei, he'd almost pulled his son aside to ask that he keep an eye out for someone dearly loved. He'd smothered that idea just as he does the wandering thought of asking Kisuke to open a lone pair of doors for him (every few nights or so) when the wind blows a certain way and acting like his cheery self is not enough.