It's a bug that existed in the base TS2, and was fixed then by Pescado, by the hack "carpoolfix". I had it all the time, when General Grant brought a friend home - saw only his/her plumbob.
I know that in later EPs this bug was fixed, because I don't have anymore this hack, nor unvisible sims.
In BV this event returned; when a teen runs away from my house, and I call the police - he returns invisible. Seems that I have to put again the "carpoolfix" hack.
http://www.moreawesomethanyou.com/ffs/bv/hacks/Scroll to "carpoolfix" (it's for BV)