Presumably it's air source and if it's indoors it will just be the air in whatever room it's in. That's how my ventless dryer works. I'm not sure what the implications are for taking heat from air that may have been heated by another heat pump are for efficiency. But also if it's summertime, they may be relieving load from your air conditioner.
> That's how my ventless dryer works.
Do you mean a heat pump dryer? Those aren't taking heat from the room; they work by sending the air inside the unit through a powerful dehumidifier. (I have one, it's very nice.)
A dehumidifier is the same thing as an air conditioner. The coolant coils are still just exposed to the ambient air in the room. The same way a window air conditioner drips moisture, the dryer is like a backwards air conditioner that blows cold air into the room and hot air into the drum and condensed water drips down the drain. I suppose it ultimately doesn't matter once you open the dryer door and everything mixes together.
My heat pump dryer does not blow cold air into the room. (My heat pump water heater does.)
The air inside the dryer is cycled through a dehumidifier and then the water is pumped out into a drain. This is in contrast to a typical US dryer that just blows hot air into the drum and then out a tube outside. Apparently most of the world doesn't do it like we do.