So then we are back to my original qustion, "could it be a ploy". The assumption you are making is that the only thing he will do/has done is retrieve the heroin.
Please listen to me. I am not
saying that is the only thing he has done. I am saying that is the only thing we have seen him do.
If he did anything more, we do not know about it yet
, because we only know what we were shown in the episode. And the only conclusions we can make at present are based on what we have actually seen up to this point.
There is an enormous and fundamental difference between saying "We have no evidence that it happened" and saying "It did not happen." And if you can't recognize that difference, then we are at an irresolvable impasse.
An action that does nothing by itself unless we theorize on why he retrieved it. It may be the start of a downward spiral as he starts using again. OR, it may be used as a prop in order to convince Watson that he is using again, prompting her to decide not to move out. A result he has shown to very much prefer and would fit in with his exhibited use of manipulation. It would fit the character.
But we have no evidence yet
to suggest he plans any such performance. Again -- I am not saying "I know he will not do this." I'm only saying we do not yet have evidence that he will do this. If he does, we won't know until we see another episode and are given more evidence, and that won't be for several months. Until then, we have no reason to favor that hypothesis.
But again, I think we have a very solid reason to doubt it -- not based on anything to do with Watson, but based on a simple understanding of what drug addiction is. At every moment of Sherlock's life, he is tempted to use drugs, tempted in a way that non-addicts cannot fully comprehend. If you see an addict hoarding a drug, or taking it out of his hoard and putting it in his pocket, the most probable explanation by an overwhelming margin is that he's on the verge of a relapse. Even if he decided to indulge that temptation as a ploy to make Watson stay with him, that would just be an excuse for acting on the very real craving he can't help but feel. It would still be a genuine relapse regardless of how he justified it. Heck, I imagine many relapses are based on some kind of rationalization or excuse.