Disclaimer: I don't own Bleach.
Shinji is old enough and observant enough whether he puts these observations to use or not. So within the little time Ichigo stayed with him and the other Visored, coupled with the first month of living as a couple, he noticed certain things - the way Ichigo liked to take long hot showers, the days he seemed cold in the warm weather, how he sometimes fidgets and shifts unconsciously. But he just chalked it to the participating in the war, the loss and return of two parts of Ichigo's soul and the time he spent alone for two years.
And didn't Shinji loathe those two years - trapped in the Soul Society, away from Ichigo and trying to find a way to restore his powers and when Kisuke found the way, they had to convince the shinigami to help. Kyōraku may be a bastard but he at least was reasoned with and if not, then he would have definitely lost half of the Gotei. Not that this had stopped them before. And they even had the gall to try recruit him again. Shinji couldn't help but sneer at that arrogance. They dared use Ichigo and throw him away when he wasn't needed. Just like they did to Shinji and the rest of the Visored.
Soul Society may have been home but now it brings nothing but hate to him. He would feel nothing but satisfaction if it burns to the ground. Though he was surprised the ever stoic Kuchiki taicho and a scowling Unohana taicho stood up for Ichigo. (And he is a little terrified - Ichigo was able to sway Unohana, one of the oldest Shinigami, to his side. The vindictive side of him and Sakanade purr at the chaos that this could result to.)
The moment they had the sword ready to restore Ichigo's powers, he took Hinamori with him and later left her with Kisuke and Yoruichi. The poor girl was so unstable and being a Lieutenant at the Fifth didn't do her any good. He doesn't want to be a captain again but he couldn't leave a child like her to wither because of Aizen. And also Ichigo with his bluntness would be good for her and quickly boost her confidence.
So yes, Shinji is old and observant and smart but he is also a petty bastard. He noticed the many times Ichigo held onto him like a lifeline, how he unconsciously touched Shinji whenever he could, how he seemed to thrive every time Shinji threw an arm around him, how the random fidgets started to decrease. He noticed how being utterly alone for two years left his boyfriend surprised at many day-to-day things. Shinji notices this all, remembers what he knows of Ichigo's childhood up to his teenage years and feels his blood boil as pieces click together. Shinji is petty and rightfully furious so the first thing he does after Ichigo moves in with him (and after showering him with affections and apologies for not returning sooner) is to find Isshin and beat him black and blue. It's nowhere near enough to make up for what the bastard did to Ichigo for all these years but only beating him up would not help Ichigo deal with the touch starvation. So Shinji returns home and cuddles with Ichigo for the rest of the day.
Which leads him to where he is now, finally home after talking with Momo-chan, only to find Ichigo curled up in a ball, as if cold, on the coach, wearing something suspiciously similar to one of his dress shirts and trousers. His sleeping face is peaceful, no sign of the ever present scowl, making him look impossibly young.
Shinji blinks. This is... they have certainly used the other's clothes before but now when he thinks about it, Ichigo has really taken to wearing his clothes. It was cute.
A smile creeps on his face and his hollow part purrs. This was good. It has been nearly four months since they started living together and seeing Ichigo so relaxed, covered with his scent, does wonders to his mood. He sits carefully next to Ichigo and runs a hand through his hair. He stirs but leans in and Shinji loves how relaxed he is. Ichigo is a war veteran and Shinji knows nobody leaves a war unscathed, knows how bad he had it with nightmares, how bad he still is. To have him so open and trusting is one of Shinji's greatest joys, one he would never let go.
"Mmm, Shinji? You back?" Sleepy brown eyes look at him as their owner gets up and rests his head on his shoulder and snuggles closer.
"Awww, ya' could have told me ya' wanted cuddles, my dear Ichi-chan~!" He sing-songs, one part teasing, one part really wanting to hug the hell out of his adorable boyfriend.
"W-wha- I, no! I don't-" Red creeps on his face as sleep leaves and Ichigo sputters but Shinji quickly scoops him up and goes to their bedroom. They both fall on the bed and Shinji laughs and laughs as Ichigo scowls but couldn't quite stop the quirk of his lips and settles in Shinji's arms. Ichigo smells like home and Shinji hasn't been happier.