I got a notification saying that an AirPod (yes, a single AirPod) was detected near me, but I do not have it and it won’t let me locate it either.

Apple Support has an article online (link below) that may be helpful:

What to do if you get an alert that an AirTag, Find My network accessory, or set of AirPods is with you

If you borrowed something yesterday ...maybe an AirPod is with that

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Re: Is it possible to deal with it even when you don’t have it and find my can’t detect it?

Article says:

(if you scroll down under "Get information about or disable an AirTag, Find My network accessory, or set of AirPods" heading in above article)

"To disable the AirTag, AirPods, or Find My network accessory and stop it from sharing its location, tap Instructions to Disable and follow the onscreen steps.

After the AirTag, AirPods, or Find My network accessory is disabled, the owner can no longer get updates on its current location. You will also no longer receive any unwanted tracking alerts for this item".