Do DC adapters consume energy when no device is drawing DC current?

This really depends on the inner workings of the individual adapter. A cheap one might just continue running, while a more elaborate one will turn itself (almost completely) off.

For example, Nintendo's power adapters for the Wii U and 3DS are basically dormant when not connected (the current is simply too low for measurement; at least in my case).

If you're curious, you can buy an energy meter, which is like a plug you put between the outlet and the device you want to measure. It will tell you the current amount of power consumed as well as the consumption over time.

All power adapters have some parasitic loss inside them such as:

• switching loss from hysteresis of the transformer
• switching loss from transistors (leakage current)
• Partial conduction from movs/x/y caps (age/wear)

However like Mario has advised, they do ramp up/down subject to demand, but only for smart chargers like your laptop and not so much for basic devices such as wall warts.

I hooked up a Lenovo 65-watt adapter to a good Wattmeter and it drew less than 1 watt after the first "surge" of plugging in (2 watts). I tried an Apple adapter and it did not even register.

Ordinary good quality adapters do not draw appreciable current and can be left plugged in without any concern.

I trust this helps.