Michael
I assume you are selecting from the Cars entity. If so, try something like this:
[Rencars.Cars._User=$currentUser and CarReturned = empty]
Since CarReturned is an attribute of the entity you are selecting from, there is no need to qualify it.
Hope that works - Mike
You can do this using the NOT operator, see https://world.mendix.com/display/refguide4/XPath+not