When did Shinobu enter Araragi's shadow in Tsubasa Cat?
Araragi did go check out Mister Doughnut store Mayoi was talking about, about five minutes into Tsubasa Cat Part 4. 




He didn't find her, but this seems like the most plausible time for her to sneak into his shadow without him knowing.

In case of the alternate timeline, Araragi didn't know about that Mister Doughnut, didn't go there and didn't get Shinobu into his shadow.