Clever thief wants to lead any investigations away from him and onto the innocent 3rd party
There are better ways to launder stolen bitcoins. And if the thief will be caught, he will be prosecuted anyway in most cases.
The only way to be 100% secure is to not give your key to anyone else, ever and only decrypt on your own machine.
I assume that my data is never 100% secure, because any today's computer system is way too complex to be absolutely sure about anything. But of course, such practices, combined with strong passwords, can make you at least 99.99% sure.