That is an excellent question.
The baptism of fire, which John promised
Christ would bring, means that when men receive
the actual companionship of the Holy Spirit, then
evil and iniquity are burned out of their souls as
though by fire. The sanctifying power of that member
of the Godhead makes them clean.
Old Testament Student Manual, Enrichment D (Festivals)
http://ldsces.org/inst_manuals/OTInstGen_2Sam32489000/Chapters/OTInstGen_2Sam32489000_25.pdf
Receiving the gift of the Holy Ghost is sometimes called the baptism of fire. "For the gate by which ye should enter is repentance and baptism by water; and then cometh a remission of your sins by fire and by the Holy Ghost" (2 Nephi 31:17.) This "fire" from the gift of the Holy Ghost cleanses us from our sins when we repent and is what purifies and sanctifies us.
Gift of the Holy Ghost
When the baptism of fire was betsowed in Helaman, Chapter Five, it was not unnoticable:
44 And Nephi and Lehi were in the midst of them; yea, they were encircled about; yea, they were as if in the midst of a flaming fire, yet it did harm them not, neither did it take hold upon the walls of the prison; and they were filled with that joy which is unspeakable and full of glory.
45 And behold, the Holy Spirit of God did come down from heaven, and did enter into their hearts, and they were filled as if with fire, and they could speak forth marvelous words.
46 And it came to pass that there came a voice unto them, yea, a pleasant voice, as if it were a whisper, saying:
47 Peace, peace be unto you, because of your faith in my Well Beloved, who was from the foundation of the world.
48 And now, when they heard this they cast up their eyes as if to behold from whence the voice came; and behold, they saw the heavens open; and angels came down out of heaven and ministered unto them.
49 And there were about three hundred souls who saw and heard these things; and they were bidden to go forth and marvel not, neither should they doubt.
(Helaman 5:44-49)
13 And it came to pass when they were all baptized and had come up out of the water, the Holy Ghost did fall upon them, and they were filled with the Holy Ghost and with fire.
14 And behold, they were encircled about as if it were by fire; and it came down from heaven, and the multitude did witness it, and did bear record; and angels did come down out of heaven and did minister unto them.
(3 Nephi 19:13-14)
So, hopefully, someone can explain why it was unnoticable in the particular instance mentioned in your post.