I’m with asyyeger on this one — if I have to go back and get something after I pay, that’s what I do, just to prevent any mis-impressions.
Once it’s done with however, I don’t think you really have to worry, because as other posters said most likely people realized what happened. Often in stores with self-service drinks, individuals will take a drink after they pay.
As for your other point, I have never heard that there is an issue of Chillul Hashem just because your actions seem “not normal” (assuming they are not objectively negative actions). Shabbos and Kashrus are “not normal”, too, but there is no Chillul Hashem in publicly doing it! Certainly tznius is not normal, but we don’t worry that when we walk in the streets in the summer goyim are thinking “are they normal to be dressing that way?” (And trust me, that’s exactly what they are thinking!) On the contrary, publicly performing miztvos is considered a Kiddush Hashem. I am not addressing whether you should have gone back (I think there is enough grounds to assume no Chillul Hashem was made), but just disagreeing with this argument that being viewed as strange would be in the category of Chillul Hashem.