Your right it can't find it but what I want you to realize is:
The rule is "if you are the destination reply".
"if you are a router on the network who happens to have the mac address reply"
My point is this if the host doesn't already have the mac address mapped it needs to arp and the only one who is going to respond is the destination. Because the destination is not on the same lan and therefore cannot reply it will cause a destination unreachable.