One important thing to know: Does the notification just let you know that an AirTag is nearby OR is it telling you that it is also "following you."
The former would be "normal," as this is how the interaction between an AirTag and an iPhone that is not associated with it as both, by default, participate in Apple's Find My network. This is by design
The latter would be more concerning, as in that case, the tag is not only near you, but is either on you or in something you are carrying, or in your vehicle if you are driving. The tag's presence could indicate, at least, two things:
- It was left there by mistake, OR
- Someone intentionally placed it there potentially because they want to track your location.
In either case, chances are that it is located such that it may be hard to hear the sound. Worse, those determined to track someone can disable the AirTag's speaker so that it won't play a sound at all.
Regardless, if you have an iPhone 11 or newer, you can use "precision finding" which would help locate the tag within inches negating the need to have it play a sound.
Ref: What to do if you get an alert that an AirTag, Find My network accessory, or set of AirPods is with you - Apple Support