A small, ordinary looking red pouch with plain drawstring.
At first look, or under the eye of a hurried thief, it would appear that there’s nothing special about it. But when opened, even by the owner, metal threads on the pouch’s drawstrings scratch against hidden metal plates in the cloth and emit a horrible, ear-peircing screech (like nails on a chalk board), almost instantly awaking or drawing the attention of the owner, most useful for detering theives or pickpocketers. This effect can also be magical, see below.
If you decide to make the pouch’s effect magical, then simply make it that when opened by the rightful owner, it does not screech.