^ It happened. I think people didn't comment on it because, in the grander scheme of things, its a small character nuance.
Since you weren't aware of this having happened though, here's the specifics:
Watson wakes up late and therefore misses checking Holmes out of rehab (she finds out he left on his own, effectively staging a jail break), so she goes to his house, getting there in time to see some nameless woman putting her clothes on through the open window. After the woman leaves, Watson goes inside and Holmes does his awkward 'pickup line' recitation that was in all the early trailers.
Watson introduces herself and confronts him about the rehab jail break and the woman, and Holmes is nonchalant and dismissive on both fronts, talking about the two things the way you'd talk about the weather, describing things with the woman exactly as Christopher said.
Watson later calls BS on the thing with the woman, using it to get Holmes to all but admit that he's running from the event - which she deduces involved a woman - that led to Holmes' drug crash in London.