@lobsterboy Thanks for the correction. I am not really good with words

But still, there is no sign of any survivor and zombie, which is very questionable. And the point that you mentioned, about the two soldiers, it was a movie. So, there could be a chance that, he might have seen it in the past before, like in his childhood.
The footprint however, is still a mystery for my assumption. The world being abandoned with only two of them is very suspicious.
While writing/typing this, I thought of another theory that, this could possibility be an experiment with the girl and him. Like how special she is, to still have human emotions.
But, I would just end this here with stating that I was only 80% sure, so there is still a chance of this assumption being wrong.
And I am just working out too much from just 5 chapters so far. We will see, what's the case is all about in future chapters (hopefully). But I am sure it's not going to be a good reason behind all this.