A Stethoscope can be bought from Martin Thwait at the Rogue's Den. Level 50 Thieving and Agility is required to access Martin Thwait's store and purchase the item. You can also obtain the Stethoscope without 50 Agility by having 99 Thieving.
A stethoscope increases the chance of successfully cracking the Wall safes in the centre of the Rogue's Den main chamber, near Martin Thwait; or the wall safes in the Thieves' Guild (Note that you do not need to "Use" the stethoscope on the safes, it just has to be in your inventory). A stethoscope, however, has no effect when cracking the safes in the Rogues' Den activity itself or for any other Thieving activity, with the exception of the Wall safes in the Rogues' Den.
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