In my experience, the Books of Residents of towns can often
refer to individuals who had no other vital records recorded
(to my knowledge in Poland). In fact, though I found many
records tracing back to my 5th great grandparents, it was
the birth record of my own great grandfather, who came to the
US and died only a few years before I was born, that was missing.
However, eventually a paid researcher in Poland found him in the
Could someone escape being mentioned in a BoR? I imagine, yes.
Though I often see things like "missing, left for the United States"
or "missing, escaped military service" -- so, I think someone did
try to record everybody.
If an archivist told you years ago that a relative had no records
in their archives, I wouldn't trust it.
Unless this was someone you paid, and they told you what they did
and did not look at, I'd ignore it. Looking for names in all
appropriate records takes a lot of work.
I can't imagine that a typical archivist would (or even be
allowed to) do such an expansive search.
As was said previously, I wouldn't look at it as "your only hope",
but, perhaps, the best hope that you have not considered in depth