Thanks all for the summary. I just started in a group as player (yipee) and they play second edition (oh well...). In the second edition the nobody here spell explicitly states that the victim of the illusion realizes what has happened after the spell expires.
This really limits the spell for any cloak and dagger action infiltrating an enemy castle or similar. (it would be over after a couple of minutes)
So good job 4th edition to have changed that small but important aspect.