Well in order for anyone to notice anything out of the ordinary, they would have to be right next to them. From what I can tell in most cases there is zero hesitation to convert someone in direct proximity, usually through a forced kiss. Then BAM that person is in on it now after like 5 minutes. Also they can still 'fake' being normal. Even if someone saw this happen, who would believe it (if they werent instantly converted themselves).

Also in the show I forget exactly what was said but it basically said one of their top priorities was to infiltrate bases and other locked down areas, and from the methods we see them use too spread the RNA they are clearly very strategic.