It's not the same as "smelling" it, and as far as I can find, there are no studies testing this ability, but there are countless stories about cats (and sometimes dogs) who saved their human families (and dogs) from carbon monoxide poisoning.
In these cases, the CO levels were high enough to cause severe symptoms in humans, but they were asleep and didn't notice. The cat noticed and woke at least one person up.
...The time around 1 a.m. when everyone was sound asleep.
“All of the sudden Gracie, I heard she was pounding, knocking,
knocking, knocking at the door,” Shanahan said. “And so I got out of
bed and to stop her from pounding at the door, and I looked to my left
and Annette was there in the chair.”
“I was hanging onto the arm of the chair, and I thought I was dying,”
his wife Annette said.
“And she called 911, and all she could say was ‘can’t breath.'”
When firefighters arrived, they discovered lethal levels of carbon
monoxide in the home. It was caused by a malfunction in the hot water
Annette and Kevin were taken to the hospital, where they spent the
Had it not been for Gracie — well, they’d rather not think about that.
There are many more stories. In several the family has a working CO alarm, which sounds after the cat has already woken them.
We humans can detect CO quite well. But we've been conditioned by a couple centuries of living indoors with combustion to ignore mild symptoms. Even manufacturers don't make detectors for low-level exposures that may not be immediately life-threatening, but certainly do affect your health.
Most studies are about acute exposures to high levels of CO, at least 30-50 ppm over a few hours (and all the charts about "safe" exposure levels say "for healthy adults"). But health effects can occur with chronic exposure to low levels, like 10 ppm, especially in children and people with pre-existing health conditions.
Evidence that exposure to low concentrations of carbon monoxide can
affect a number of organ systems is accumulating. It is, perhaps,
easiest to explain effects on the heart in subjects with incipient
myocardial ischaemia. Less easy to explain are effects on the central
nervous system; that these effects may not be accurately predicted on
the grounds of blood COHb concentration does, however, seem
increasingly clear. Whether long term exposure to low concentrations
of carbon monoxide can produce long lasting effects on the brain does
not yet seem to be settled. If such effects do occur, the impact on
public health may be large: many homes are heated with gas appliances
and a significant number by solid fuel; failures are inevitable and
known to be common, and thus a significant number of people must be
being exposed to levels of carbon monoxide in excess of those found in
ambient air. Even if only a modest proportion of those exposed
sustained effects, the impact on public health may be significant.
These findings may have implications for the setting of occupational
exposure limits. The Health and Safety Executive recommend a limit of
30 ppm, which can cause COHb levels to rise above 2.5% in less than
one hour. However, it should be noted that the evidence for low level
effects of carbon monoxide does not arise from occupational exposure
studies. The patterns of exposure of people exposed in their homes may
be quite different from those exposed occupationally.
We humans may not recognize CO as such, but we do usually know something is wrong with our bodies. Most people ignore that or get belittled by doctors when they try to get help. Or they might get a diagnosis that doesn't acknowledge the source, or even attempt to test for it. If tested, it's dismissed as an issue, because the prevailing wisdom is that CO below 30-50 ppm isn't dangerous (in some cases, the thresholds are even higher).
Animals can also be trained not to "bother" humans with complaints. But generally they know something is wrong and don't care what humans think about it. Many will go to great lengths to alert their humans as well.
In your creature design, take animals that have intelligence levels similar to cats and dogs and add in something specific that CO can do. If it's a world with lots of CO around in pockets, this could be something an animal evolved to detect easily.
For example, have the lack of blood flow CO causes that turns gums red also produce heat. Uncomfortable levels of heat or a burning sensation. It doesn't matter what, as long as it's exclusive to CO exposure. This can be inside the mouth and nose. Or it could be on the paw pads or anus or someplace that other animals can see the change to bright red (make sure their eyes can detect that color change). The animals might also be able to touch noses to noses (or butts) and feel the heat.
These are all signs to raise the alarm, scruff the children, and get the hell out.