I wrote in a previous post that detecting an absence means, for authors dealing with Mīmāṃsā and Śrauta Sūtras, to look for a substitute for the absent element.
Although it might be easy to say why absence is related to substitution, the opposite is not the case. Why is not it the case that one just explains substitution by saying that X becomes Y? Why do we need to refer to the absence of X, so that, instead, Y is used at its place? Could not it be that the speculations on ritual hermeneutics tend to be spatially oriented, so that spatial explanations are preferred over temporal ones?