What happens when there's some attribute or idea in the Source Domain that has no equivalent in the Target Domain?
Concept13 has no corresponding node in the target domain.
This is probably no big deal. Consider Driving in the Fast Lane on the Freeway of Love. A fact about driving in the fast lane is that, if the freeway is busy, you're likely going to have a hard time getting to the exit ramp.
Unless your *goal* is to find a mapping for everything in the source domain, you'll probably unconsciously ignore it. It'd be silly to fret over what an exit ramp means for love.
... unless you're thinking pretty hard about the metaphor. Then you might think something like "Well, you take an off-ramp to get off a freeway when you've reached your destination. Maybe that means it's time to settle down and get married? But what does the crowded freeway mean? Maybe that marriage will require you to leave your fellow journeyers behind? Getting married will change your friend group? (They'll leave you behind as they hurtle onward?)"
Paintbrush as Pump might be an example of this. Saying, "Say, a paintbrush is a kind of pump!" prompts the question "How?" Which asks you to enumerate the elements of the Source Domain:
> Paint corresponds to the water a pump pumps. > ... > Pumps have a channel through which water flows. What corresponds to such a channel in a paintbrush?... Eureka! > ...