Under the front of the automobile? If in the passenger cabin would have been sufficiently concentrated? Thirty-eight others affected in the incident—by proximity?
Some suggestion is that it was in the air ventilation system of the car (yet to be concluded on that idea). It would have gotten on their clothing, and as they walked into the pub and were around others....maybe dispersed to the 38 others.
If you were looking at some really sloppy way of terminating a target, and involving a lot of extra people...then fine. The more they dig into this...the less likely I think Putin or his folks approved this, but then the real question is....do safeguards exist over the stuff in their possession and the answer is....no.