On a trip to licence my car I had to empty my packets into a basket for the X-ray machine, and had a small knife I had forgotten. This was placed in an envelope and signed for later return. Now, it seems to me this could be the basis of several effects.

1) the basket would hold a variety of items, coins, keys, a shopping list, pen, etc. These would be placed under the spectators' noses and they could select an interesting item for you to perform with -- thus engaging the audience and getting around any inspection issues.

2) the knife could be of the color-changing variety. When you get the envelope back it is different one then placed inside, and the story continues.

3) your pockets would have been allegedly shown empty, so any later steals or swithces will be less suspect.

4) a money clip with bills and business cards can allow for "out to lunch" effects.

5) you can blame repeated trips though the "machine" for strange happenings.

Sounds like you just stumbled upon a brand new parlor routine for you to put together. Could also become a decent stage routine depending on which items were in your pocket.

2) A self-contained switching envelope would eliminate the need to switch envelopes in order to switch knives.

Are you familiar with "Contents of the Dead Man's Pockets" by Jack Finney? He's one of my favorite short story authors. There could be roots for a magic plot similar to your idea.

Pat page had an intresting way to show a empty pocket.