Internet search question...

[SnakeByte]

I attempted to search for a subject with a 16 character string (just a series of numbers and letters) under "Subject", but NBP came up with zero results.
A nzb search site however found it just fine and when I downloaded the generated nzb file, Newsbin pro opened to show the subject contained that exact string I was searching for. I was able to download it without issue. The filename also contained the string.

I also attempted to search by the poster of that file, but that also came up empty.

What are the limitations of NBP's internet search? The file was password protected.