Hello All! Hope you guys are doing great!

I just opened up a carbon monoxide detector that is a few years old. It is NOT from the 1980s etc.
When i opened it, I saw the CO sensor inside of it. The pic of the sensor is below.

My question to you is: How does the sensor sniff the CO? There is no hole nor any inlet opening. It seems to be as sealed up as a regular AA battery.
I have seen gas sensors before and they all have some sort of hole/mesh where the gas goes into. But this guy is sealed like a hot dog.
Can you please tell me how it works and how does it sense the CO in air?

Oh, yeah, when I heat the outer casing up, there is a voltage spike that I observed on a multi meter.

Thank you for your replies!!

 

There's a tiny hole in the electrode at the large end. Described here: https://vintageteardown.com/teardown/garrison-carbon-monoxide-detector/