Short Answer: Among the early appearances of "life signs" in Star Trek, there are many that are ambiguous. However, there are no early uses that appear to be mass constructions, and several (including the second overall appearance in the franchise) that are definitely count constructions.
The first appearance of "lifesign(s)" or "life sign(s)" in the Star Trek television shows and films came in Star Trek II: The Wrath of Khan. (I am restricting my attention to the visual media, since there were a huge number of novels written during the period in question, with little to no central organization of their content or made-up science fiction terminology.)
SAAVIK: Indeterminate life signs.
KIRK: Phasers on stun. Move out.
In this instance, it is ambiguous where the plural "life signs" should be construed as a count quantity or a mass quantity. Saavik (albeit played by a different actress) provides the second instance as well, in The Search for Spock.
SAAVIK: We have found the life sign. It is a Vulcan child, perhaps eight to ten Earth years of age.
This, using the singular, clearly makes "life signs" a count noun. The Klingon second officer, Torg, uses "life signs" in what appears to be a count construction as well.
TORG: There are life signs on the planet, perhaps the very scientists you seek.
It is used one more time near the end of the movie as well, but in an ambiguous construction.
In the Star Trek: The Next Generation era, the terminology first appears in the episode "Heart of Glory," where it is used extensively, but never in a way that is determinative of whether it is a count or mass term. It's the same in the next episode, "The Arsenal of Freedom." However, the next episode produced after that one (although there was one broadcast in between) was "Skin of Evil"—famous for featuring Tasha Yar's death—which features a probably count use.
PICARD: Doctor, what is the state of the shuttle crew?
CRUSHER: We're still receiving faint life signs, but the sensor readings are fluctuating. They may not be accurate.
However, the next appearance, in "The Child," the first episode of the show's second season, uses "life signs" in a different way.
PULASKI: I'm losing life signs.
TROI: You must save him.
This use is unlike all the Star Trek usages that had come before; it is actually a completely different sense of "life signs." In fact, this appearance is simply the normal, real-world use of "life signs" as a medical term—with multiple signs for a single patient. When the medical patient is at the brink of death, there are multiple ways of ascertaining their condition: heartbeat, respiration, brain activity, etc. These are all "life signs," but it is not the same sense as "life signs" (indicating living bodies) detected from a distance by fictional scanner technology. (There are also many more instances of the more common term real-world "vital signs" in various Star Trek episodes.)
An absolutely unambiguous count use in The Next Generation finally appears a few episodes later, at the very beginning on "The Outrageous Okona."
WORF: One life sign aboard, sir. It appears to be humanoid.
I stopped scanning the scripts at this point, having found such an unambiguous example. Overall, there were many more ambiguous instances than definite ones, but up to this point (covering the original Star Trek, which never used the term, the animated follow-on series, which never used it either, the original series movies, and the first several dozen Next Generation episodes), there were no instances where a count interpretation on "life signs" was impossible or even looked unlikely. So there is no reason to think that "life signs" was ever originally intended to be anything other than a count noun.
(Purely for the sake of completeness, I should point out that the science fiction use of "lifesigns" did not originate with the Star Trek franchise. However, as far as I can tell, the first prominent appearance of the term came from Richard Wilson's "Inside Story" in the June 1955 issue of Galaxy Science Fiction. Wilson seems to be mostly forgotten today, but he was a prominent writer in the middle of the twentieth century and won a Nebula award.
"Dr unheard skipper story un-
has own knwldg life Marsmoons
but wld spose if existed b totly
alien life as knwn Mars-Earth
owing uttr lack air infintsmal
grvty outpointg too sevl explore-
trips unfnd lifesigns..."
It is obviously not really possible to conclude anything about the grammatical construal of "lifesigns" for these garbled notes that appear in the story.)
