I think there may be a little mixup between the Warmachine and Hordes mechanics here.
Warjacks do not need to be in a warcaster's control area to spend focus. In the base rules, they only need to be in the control area to be allocated focus from the warcaster's focus pool and to channel spells. A Seether's Soul Drive will allocate one focus if outside a warcaster's control area because the focus is not coming from a warcaster. And it can spend it however it likes too. (Note that since Soul Drive uses the word 'allocate' it does contribute to the 3 focus from 'allocation' limit)
In Hordes, on the other hand, you do need to have the warbeast in the warlock's control area to force it to boost, gain additional attacks, etc.
I don't have my book on me for a page number, but you can see how this works in the Warjacks chapter and the "Allocating Focus Points" and "Control Area" sections of the Warcasters chapter.