I have noticed that my caffeine-ng in the tray is seemingly enabling/activating at random, even though I had virtually nothing running and I had no apps listed explicitly in the Preferences dialog:

The caggeine Preferences dialog without any explicit apps

After running caffeine from the command line, I could see the following:

INFO:caffeine.core:Audio playback detected (sd_dummy). Inhibiting.
INFO:caffeine.core:GnomeInhibitor is applicable, state: True
INFO:caffeine.core:XorgInhibitor is applicable, state: True
INFO:caffeine.core:XdgScreenSaverInhibitor is applicable, state: True
server does not have extension for -dpms option
INFO:caffeine.core:DpmsInhibitor is applicable, state: True

The first line might be the culprit of the problem. By searching I've found that sd_dummy is related to the speech-dispatcher package. On my system it was installed as a dependency of orca, which I do not use.

Solution 1: User-only autospawn disable

Create a local configuration directory, in case it is not created yet:

mkdir -p ~/.config/speech-dispatcher

Add the directive there:

echo "DisableAutoSpawn" >> ~/.config/speech-dispatcher/speechd.conf

This might be the least invasive solution.

Solution 2: System-wide autospawn disable

This is the variation of the above, but uncomment the DisableAutoSpawn in the /etc/speech-dispatcher/speechd.conf or put it there if it is not present.

Solution 3: Uninstall speech-dispatcher

The most straightforward solution is to just remove speech-dispatcher package from the system:

sudo pacman -Rnc speech-dispatcher

The above also took orca with it, but your mileage may vary. Hope it helps!