Annoying rather than alarming. Sessions do not last for ever, and signing back in is the whole fix in almost every case.
The ordinary reasons
A session expired. They do, eventually. Nothing is wrong and nothing is lost.
You cleared your browser data. Metzee runs in your phone’s browser, so clearing site data signs you out of it like any other site.
Somebody used Sign out all. If you have ever signed in on a device you did not trust and later ended every session, that includes this one — Signing out everywhere ends every session, on every device.
A different browser. A link opened from WhatsApp or Instagram opens in their in-app browser, which is a separate place from Chrome or Safari and has its own session. You are not signed out; you are somewhere else. Opening the link in your normal browser is the fix.
That last one accounts for most of these.
Getting back in
However you signed up: your phone number, with a WhatsApp code; or your email and password; or Google.
If you signed up with a phone number, you may not have a password at all — the security screen says so: You sign in with your phone number — add an email below if you like. So "forgot password" is not your route; the code is. My OTP is not arriving
What you have not lost
Nothing. Your orders, bids, wins, saved items and credits are on your account, not in the browser.
Including a win with a payment window running. That is the one urgent thing: if you were signed out around the end of a show, sign back in and check your orders, because a clock may be running on something you won. How will I know if I won?
When it means something
If it keeps happening on the same device, every day or two, something on that device is clearing its storage — a cleaner app, a privacy setting, or private browsing mode. Metzee cannot keep you signed in through that.
If you did not do it and cannot sign back in, treat it seriously. Check Signed-in devices once you are in, and use Sign out all to end every other session. Then add an email if you have not, so you have a second way in.
The thing worth doing now rather than later
Add an email to your account, from Security & account. It gives you a second route in and somewhere to send a reset — and if your phone number ever changes, it is the difference between an inconvenience and losing the account.
What nobody will ask you for
A sign-in code. Metzee will not ask, and a seller has no reason to. Anyone asking for one is trying to sign in as you.
More: how to create an account, can I have two accounts, how to delete your account.