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This warning is somewhat different than the other questions I've seen on this site, in that only a colon and now bibtexkey is mentioned. It doesn't cause an error and seem output seems fine to me, but I'm interested in what I would need to clean up to make get rid of this warning.

I use \usepackage[numbers]{natbib} and \bibliographystyle{unsrtnat}.

When I run BibTex, I get Warning--I didn't find a database entry for ":" after the usual output. I've checked by searching for {:} in my document to make sure I am never citing anything by this bibtexkey, like \cite[...]{:} or \cite[...]{:} and did not find anything. But that would also cause undefined citations, I believe, because it would certainly be a typo.

What might be the issue?

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    Look in the .aux file whether you find \citation{:}
    – egreg
    May 7 '20 at 14:43
  • there is no \citation{...} of any kind, but lots of \bibcite{...}'s though. No \bibcite{:}, however May 8 '20 at 18:08
  • Hmmm, BibTeX only searches for those entries for which it can find a \citation{<entry key>} in the .aux file (unless an entry is referenced in a crossref field of another entry for which there is a \citation; \citation{*} automatically applies to all available entries). If there are no \citations at all, BibTeX shouldn't even try to look for citations, indeed it should complain I found no \citation commands. ... So I guess what I'm trying to say is: please try to throw together a small example document that reproduces the message.
    – moewe
    May 9 '20 at 11:55
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    thanks for your help, but, by accident, I found a typo that I had made and corrected it May 9 '20 at 13:39
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If I try with

\documentclass{article}
\usepackage[numbers]{natbib}

\begin{document}

\cite{x}

\citep[x]:

\end{document}

then the .aux file contains

\relax
\citation{x}
\citation{:}

These entries will be picked up by BibTeX in order to populate the .bbl file. Once you run BibTeX and LaTeX again, the entries in the .bbl file will produce \bibcite lines in the .aux file, such as

\bibcite{x}{{1}{2020}{{Uthor}}{{}}}

Thus you will be able to find the offending \citation{:} entry in the .aux file and guess where the problem is. A binary search can be useful to narrow down the lookup.

The warning

Warning--I didn't find a database entry for “:”

can only be issued by BibTeX and it only looks for \citation lines in the .aux file. So if the warning is issued, there is a line of the form \citation{:}.

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I came across the answer by accident: I had mistyped \citep[...]:, using square brackets instead of curled ones. After making some other changes and recompiling I noticed that there were undefined citations. To me, it seems like this message wasn't there before, but I don't know if that's plausible, so probably I had just missed it.

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