I don't understand why you make it so difficult for you, except you play solo. Talk to the to-be-victim with one char and steal with another. Slender out of view with the thief with all the time you need. Then change back to the talker and end the dialog. The victim searches your innocent group members and never finds anything. It's called distraction in the real world.
That NPCs find stolen stuff of others and even care for it is strange however. For a time my distraction group was accused for haven stolen goods on them. I searched and searched and found nothing. I was not careful enough, later by coincidence I saw that it was a bloody needle I had stolen in Act 1 for what reasons ever and long forgotten which caused my problems.
