It's most likely not the turbo, which spools down the minute the engine is shut off as there's no longer any exhaust to drive the impeller turbine. And you wouldn't hear the turbo anyway, unless compressor vanes were scraping...a catastrophic failure that would have disabled your car. My guess is you're hearing the air pump, compressor, or electric radiator fan.

To locate the sound, you could open the hood before you shut the motor off and then locate the sound at shutdown. Air pump is under the battery, fan on the radiator, turbo between the engine and the firewall.

My guess? Air pump. When functioning normally, it comes on just after start up, and intermittently thereafter, to maintain optimum operating temperature in the catalytic convertor. A whining sound is normal, and if it's running when you turn off the key it sounds just like what you describe - a jet engine spooling down. It's also a known repair item once you're approaching 100k miles.
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(98 S70 T5SE misc mods, mostly lighting) (92 940GLE)