Having had UPS lose a VERY expensive bicycle, I can tell you that it will take quite some time for them to resolve this.
In my case, they finally found the bike intact in its case after almost a month! I was very happy to get it back.
It takes quite some time for them to get misplaced packages to a central lost-and-found. Then they have to go through them and try to figure out whose package it is. Unfortunately, it is NOT a fast process.
However, there is the very real possibility that in the end you will get your bass.