Prior to the days of all metal Thermos Flasks, it was common for the interior of these devices to consist of an all glass container. Having the air evacuated from between the double layer of reflective glass, ensured that there was effective insulation of the contents, thus retaining either heat or cold.
disaster with these flasks occurred the instant they were allowed to drop unto a hard surface, as this inevitably shattered the inner container. The unmistakable sound of glass fragments mixed with chicken soup (or whatever was in there) became characteristic of a flask permanently destroyed in this manner.
It is this unique sound that protects the contents of the Thermos Cacophonous. Prior to leaving the precious Thermos unattended, all you have to do now is to pretend to drop it in front of the usual contents stealing suspects.
Now amid much cursing and gesticulating, it will release the exact sound of a destroyed flask. This is because you have activated a small device hidden inside the main body, which allows some loose particles to be released from the electromagnet that was holding them in place. They now rattle around convincingly, just like a broken flask.
You can even add to the illusion by demonstrating your (fake) macho powers and consuming the contents. (Thermos Cacophonous is only effective once with each new audience, meaning that it is of course a totally halfbaked idea)