I think it's the whole soul-mate thing, isn't it - that when you're with the one you really love, you have an awareness of them that even extends to a distance, and their absence is like missing-limb syndrome. So if you don't feel like you're missing a limb, then they must be still alive...

I just don't think the author's set up that kind of relationship between them yet... they're getting there, but they're both still uncertain of each other - and in fact Doyle's just resigned from CI5, come to think of it!