You found a key and have no idea what it opens

Start with what is around it

A key rarely travels alone. Look at what sits beside it: a tag, a receipt, a business card, a slip of paper with a number, a small brass token, a code written on the back of an envelope. Those clues matter more than the key itself. Take a photo of the key next to anything that was stored with it before you move things around.

Check the key for markings. Some have a number stamped on the head or the shaft. Some have a brand name, a colored plastic cap, or a shape that points to a particular kind of lock. Write down anything you can read and keep the key somewhere you will find it again.

The common places a stray key belongs to

Most mystery keys come from one of a few places. Run through this list and see which ones fit the person you are sorting for.

• A safe deposit box at a bank or credit union
• A storage unit, indoor or drive-up
• A home safe or a fireproof box
• A locker at a gym, club, workplace or station
• A post office box or a private mailbox
• A suitcase, a filing cabinet or a piece of furniture with a lock
• A bike lock, a car, a shed or a second home

Small flat keys with a number often belong to a safe deposit box or a locker. Longer keys with deep cuts tend to be for doors. Very small keys can be for a diary, a jewelry box or a piece of luggage. None of this is certain, but it narrows the search.

Look through the paperwork first

Before you call anyone, read the papers. Bank statements sometimes show a safe deposit box charge, listed as a separate line or a yearly fee. Storage companies send invoices by email or post, so check old statements and any folder of household bills. Insurance papers may list a safe or a specific item kept in one.

If you have access to their email, search for words like storage, safe, locker, box, rent and unit. Old confirmation emails often include the address and the unit number. If you cannot get into the email, the guide on being locked out of their email covers the usual routes.

Their will and any letters they left may mention a box or a unit. If you are still looking for those papers, where to keep your will explains the places people usually store them.

Call the places that fit, one at a time

Make a short list of banks, storage companies and clubs the person used. Call each one and ask a simple question: did this person have an account, a box or a unit with you? Have the key and any paperwork in front of you. Ask what they need from you to check.

Companies change their rules, forms and fees, so ask what their current process is rather than relying on what you have heard. For a safe deposit box, the bank will explain what it needs and whether the box is in a sole or joint name. For a storage unit, the company will say who is allowed access and what proof they want. Note the name of whoever you speak to and the date.

If the key is for a home safe

A safe at home is usually the easiest to open, because you already have access to the house. Try the key in any safe, lockbox or cabinet you find. If it fits but will not turn, the safe may need a code as well, or the lock may need a light oil. Look for a manual or a receipt with the safe, which often names the maker and the model.

If the safe is bolted down and the key does not work, contact the maker and ask what they suggest. Keep the contents in place until the family has agreed who should be present when it is opened.

What can wait

You do not have to solve this today. If the key opens nothing obvious and no paperwork mentions a box or unit, put the key in a labeled bag with your notes and set it aside. Come back to it after the more urgent tasks are done. The guide on the first 48 hours sets out what actually needs doing early.

One thing worth doing soon: tell the other people close to the person that you found a key and where you are keeping it. A sibling or a friend may recognize it, or may have the matching paperwork.

Write down what you learn

Keep a single page with the key's description, where you found it, who you called and what they said. If the key turns out to belong to a box or a unit, add the address, the account number and the contact name. That page saves the next person a lot of guessing.

When you have worked it out, add the answer to your own envelope so your family never has to do this search. A short note in the Hidden and valuable things section is enough.