Right here on TuD, I've read about more unpleasant experiences of this kind than I can count. For whatever reason, my experience has been pretty benign. I've never been asked to "do it someplace else", and only rarely even received odd glances. My impression, which keeps getting reinforced, is that most people are so wrapped up in their own heads (or busy staring at their phone) that they are largely obvlivious to what's going on just a few feet away.
Also, I've often had the experience of people just nodding or smiling in recognition. Not long ago I pulled out a syringe in a restaurant and the person across from me, who was also co-owner of the place, said, "Oh you're on insulin? So is my brother." And that was that.