Q:

Prevent BibLaTex from suppressing pagination prefix

I am citing a manual that has both section numbers and paragraph numbers. Sometimes, I want to cite an entire section, other times I want to also include a paragraph. For instance, I might type \cite[1.2.3 \P 4]{entry} to refer to section 1.2.3, paragraph 4. My expected output is [1, § 1.2.3 ¶ 4], however whenever I specify the paragraph number, the section symbol is suppressed. I suspect the reason why is that BibLaTeX is trying to determine whether the citation is singular or a range, and can't make sense of it.
My .bib file might look like this:
@manual{entry,
    pagination = "section",
    title = "Foo Bar",
    url = "http://www.example.com"
}

A:

It's pretty much as you suspected. For the page prefix feature biblatex determines whether the postnote argument you have supplied is a single numeral (\ifnumeral) or a range or list of numerals (\ifnumerals) in order to be able to give the correct prefix 'p.' or 'pp.'. But
1.2.3 \P 4

is neither of those because the \P is not recognised as part of a number.
You can trick biblatex into thinking \P is an acceptable part of a number with
\NumCheckSetup{\def\P{}}

Then
\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{babel}
\usepackage{csquotes}

\usepackage[style=numeric, backend=biber]{biblatex}
\addbibresource{biblatex-examples.bib}

\NumCheckSetup{\def\P{}}

\begin{document}
\cite[1.2.3]{sigfridsson}

\cite[1.2.3 \P4]{sigfridsson}

\cite[1.2.3-1.2.4]{sigfridsson}

\cite[1.2.3 \P4-1.2.3 \P5]{sigfridsson}

\printbibliography
\end{document}

gives

If this is just a one-off you can add the page prefix manually with \pno/\ppno:
\cite[\pno1.2.3 \P 4]{sigfridsson}

