It’s 10 years old, which is the usual expiration date. Google says I can throw it in the trash, but the back has an unmistakable “do not trash” symbol (the one that looks like a curbside household trash bin). Which should I pay attention to? How do I get rid of it if I shouldn’t just toss it?
I assume there are some electronics, at the very least. Do you have a local e-waste recycling service? I would give it to them.
Is it also a smoke detector? I once took an americium-based smoke detector to a hazardous waste recycling center. They told me I should just put it my household garbage. They said that such detectors are safe to go into the waste stream one at a time, but if the recycling center collected them as e-waste and put them all in one place, the concentrated radioactive material could be dangerous.
BTW, I assume you meant that this is a CO (carbon monoxide) detector, not CO2 (carbon dioxide).
Just a CO (and you’re right about that 
) detector.
You could, have, instead followed in the footsteps of a boy scout who earned his badge in nuclear reactions.
You can send them back to some of the manufacturers. I doubt any will pay for the postage. Your local hardware store, either small or big box type, may take them back for recycling. My town will take them. It’s possible the ‘Do Not Trash’ symbol is there because of the battery which is usually non-removable.
AFIK fire stations accept CO & smoke detectors
I saw a Facebook ad for a company that offered, among other things, CO2 detectors. I said, “Yeah, I have one of those and it goes off every time I breathe.”