Why does a non contact AC voltage detector detect voltage on a DC line that is not flowing
They are not detecting the DC voltage of the power supply. They are detecting the AC that is present due to leakage current in the power supply.

Every power supply will have leakage, and when the power supply is not grounded, this leakage current has no low resistance return path, and will create a voltage.
This can be detected by non contact AC voltage detectors.

Switch mode power supplies have this by design (C1), or by parasitic effect of the transformer inside.



The leakage current is very low, below a few hundred uA or even less. You might have noticed a "buzz" feeling touching some appliance, that is the result of this leakge.
Large appliances can leak a higher current, and this might give a tingle.