I had something like this happen to me when my husband was in rehab. My brother-in-law found it while he was cleaning up my husbands car (just being nice) and it really upset him. I know that I debated on telling my husband about it because he was in rehab at the time, and I figured what was the point. Obviously when he was using drugs, he had drugs. I dont think he purposely left them there for when he got out of rehab, or even if he did... after detoxing and getting help... I assumed his mind would be in a different place.
I cant remember exactly now, but I know I told him about it - but it was much later and he was either almost out of rehab, or home already. It wasnt much of a discussion, he felt bad his brother experienced it, and they may have talked about it - dont know. But other than that, what was there to say? When he was using drugs, he kept drugs around.
Do what feels right to you.