I'm not sure whether we know for sure exactly how this word is used, beyond the definition of "life signs" in the dictionary.
See also the comments on https://www.duolingo.com/comment/26717843/yInroH-nejbeH-Hoqra .
Another mysterious word is paSlogh, glossed "socks"; it's unclear what a single sock might be called, and I'm not sure whether it's certain whether paSlogh counts as singular or plural grammatically.
I would imagine that yInroH works like "irregular plurals" such as ngop "plates" (the plural of jengva' "plate") and is treated as a grammatical singular; perhaps like collective nouns in English such as "furniture" which usually refers to multiple pieces of furniture (German would use a plural, die Möbel, here) but is grammatically singular.
In addition, Klingon features a number of words that are inherently plural. These words are treated grammatically as singular. When we know both the singular and plural forms, their use is mostly obvious.
peng lubaH baHwI'pu'
cha lubaH baHwI'pu'
Notice that both sentences use the verb prefix lu-, which agrees with a plural, third-person subject and a singular, third-person object. Inherently plural words like cha are treated grammatically as singular. Consider American English group words. In British English, "the group are going," but in American English, "the group is going."
Now consider words like yInroH "life signs," paSlogh "socks," and HoSDo' "energy beings." They seem to have plural meanings, but it's quite possible their plural meanings are built in. That is, a yInroH is the complete set of readings that one obtains from the sensors, not each sign individually. If that's true, then it makes perfect sense to say yInroH lunejbeH nochmey. Even if that's not the reason, it still might be an inherently plural word.
The best thing to do is to assume the dictionary definition is right and follow it. It gives a plural translation, so treat it as inherently plural.