What personal data should never go on a tag?

Assume it is found

Personal tags are on objects that get lost: keyrings, bags, phones, cards, luggage.

Whoever finds it can read it with an ordinary phone, from a few centimetres, without any interaction and leaving no trace.

So the test for every field is what a stranger holding this object could do with it, rather than what your friends will do.

Reading also cannot be prevented. There is no way to make a tag readable by the right people only.

What never goes on

Your home address, especially on anything attached to keys. Together they are a working invitation.

Your date of birth, which with a full name is much of what a telephone identity check asks for.

Identity document numbers, account references, membership numbers used as credentials elsewhere.

Anything about somebody else: a partner's number, a child's school, a colleague's details.

And any password, for anything, including the network password on a tag that leaves the house.

What needs thought

Your full name, which is fine on a card you hand over and is more exposure on a keyring in a car park.

A mobile number, which is how a finder returns your property and is also a number you cannot easily change.

Medical information, which is a genuine tension: it exists to be read by a stranger, and that is exactly what makes it exposed.

Photographs of children, which people put on family tags without thinking about who reads them.

What to do instead

Write an address to a page you control, and put on that page only what you would accept a stranger seeing.

That gives you two things a written tag cannot: you can change it the moment the object is lost, and you can decide what it says later.

For a lost property tag, a first name and one contact method is usually enough. A finder needs to reach you, not to know you.

And keep a note of which tags exist and what each one says, so that losing one is a decision rather than a memory exercise.