People are reporting 'stinging sensations' and taking this as evidence that they've been injected... I frequently suffer from 'phantom stings and bites', where I am convinced that a wasp has got trapped in my clothing or that something has settled on me and bitten me. I'm particularly sensitive to horse fly bites (they blister) and have been known to swear that I'm being bitten when there is absolutely no evidence of biting or stinging insects - I guess my vigilence about keeping horse flies at bay makes me over-cautious.
The stinging sensation abates, there is no sign of a bite or sting and I usually forget about it in a matter of minutes. It seems to be an over-stimulation of nerves, perhaps from clothing rubbing or even just sweat. But if I were in a crowded nightclub, people brushing past me every two seconds, sweaty, itchy... and I felt this burning, stinging sensation, I can see how I could attribute it to having been injected with something, as it's a very similar feeling.
I'm not saying that this is what is happening, and I KNOW that women have a very real cause to be wary in such environments (in fact, in almost any environment). But concealing a prepped syringe and then managing to get it out, take the protective cover off the needle and inject it into a part of the body that is fleshy enough for it not to bounce off bone, then 'disappear' the apparatus where it won't be seen or felt in your clothing - it's not as easy as all that.