Could somebody track you with a tag hidden in your things?
No. A passive tag has no battery, no radio of its own and a working range of about two centimetres, which makes it useless for following anybody. To learn anything from a tag hidden on you, somebody would have to be close enough to touch your bag, at which point the tag is unnecessary.
Why it does not work
Tracking requires two things a passive tag cannot supply: a power source, so it can act when nobody is holding it, and a range longer than an arm.
A tag has neither. It is inert until a reader is against it, and the reader has to be within a couple of centimetres and stay there.
So a hidden tag reports your position only to somebody already standing next to your bag, who did not need the tag to establish that you were there.
The technology is not merely unsuitable for this. It is the opposite of what the task requires.
What is being confused with it
Battery powered trackers, which announce themselves continuously over a longer range and rely on a network of other people's phones to relay a position. Those work, and the concern about them is well founded.
They are a different technology with a different physics, and the platforms have built detection and alerting for exactly that reason.
The word tag is doing the damage here. A passive NFC tag and a battery powered locator are both called tags and have almost nothing in common.
Longer range identification systems also exist in logistics, and are again a different family, discussed on the comparison side of the site.
If you find a tag you did not put there
It cannot have been reporting on you, so the question is what it was for rather than what it recorded.
Read it before removing it, with an application that shows the content rather than acting on it, because the content is the only evidence it carries.
Take NFC Tools. On iOS it requires an iPhone 7 or later running iOS 15.6 or later, and the app must be launched manually. Background reading is available from the iPhone XS and XR onwards.
The likeliest explanations are dull: a retail or inventory tag left in an item, a tag from a previous owner, or a promotional sticker. All of those are far more common than anything sinister.
What to worry about instead
Battery powered trackers, which are the real version of this concern and which both platforms will alert you about.
The phone in your pocket, which knows where it is continuously and is permitted to by settings most people have never reviewed.
And accounts shared with somebody who should no longer have them, which is how location is actually exposed in the situations that matter.
An inert chip with a two centimetre range is not on that list and does not belong on it.
Frequently asked questions
Could an NFC tag be used to follow me?
No. It has no battery and a range of about two centimetres, so it only responds to somebody already next to you.
What about the trackers in the news?
Those are battery powered and announce themselves over a longer range using other people's phones. A different technology entirely.
I found a tag in my bag. What now?
Read it before removing it, with a tool that displays the content. The likeliest explanations are retail or inventory tags.
Can it have recorded where I have been?
No. It has no clock, no power of its own and nothing to write to. It carries only what was written on it.