In answer to the OP's question, the super-paranoid answer is yes, it is possible to have transferred ich from your DT to your QT.
The biggest danger is if you have transferred a cyst (tomant stage), and in the short-term freshwater exposure won't kill it. The other danger is transfer of the free-swimming stages (protomont and theront) and these stages probably won't have survived. The probability depends on specifics, such as if you are using the same bucket for mixing new salt water as well as removing water, and the overall time between activities. In addition, if you do water changes using a syphon and clean the sand bed during the change, thereby sucking up particles, you have significantly increased the chances of transferring a viable cyst. If you simply remove water from the water column during the change, then chances of having transferred a cyst are greatly reduced.
Possible cross-contamination of ich is mostly anecdotal and theoretical in some scenarios, and in many instances (such as in your original question) the risks are pretty low.
But, for example, I believe there is published scientific evidence that ich can be transferred between two systems just in close proximity to each other (think how far you can see evidence of salt creep!), so it doesn't take much to spread the parasite. It's your Clint Eastwood moment (Do you feel lucky?).
Bottom line is the risks of infecting your QT are low, but I personally would assume that ich is present, and take steps accordingly, as generally advised here.
