Activation lock when I'm the only owner that my iPhone ever had, asking for my AppleID iPhone 5C, iOS 9. Was bought in the Apple Store and activated by me with my Apple ID a few years ago.

Today, on 10:00 pm, when I was riding a subway train, connected to its free wi-fi, black screen with white apple appeared suddenly, when I was browsing, as if it was rebooting. It glitches like that sometimes, very rare and not very long (unlike the real reboot). But this time after the logo there was something different - on the lock screen in the notification area, there was the text "Activation required" with an icon of a circled exclamation point.

After swiping the screen, the system said that this iPhone is linked to the Apple ID, and offered me to enter my Apple ID and password, the ones that "were used to set up this iPhone". It also was showing my email with some hidden symbols. The screen was titled "Activation lock". It looked similar to this, but with my email dotted: https://phoneunlockingshopca.files.wordpress.com/2015/08/official-bypass-icloud-activation-lock-remove-icloud-account.png

I successfully logged in, and then there was an iMessage alert: "Your carrier may charge for SMS messages used to activate iMessage", as if it was the first setup. But all my data remained in place.

Then I went to https://www.icloud.com/activationlock/ and it said that my activation lock was on. And didn't say how exactly to turn it off. How to bring it back to normal?

When I google about the activation lock, everything seems to be about "previous owners" cases. But there were no previous owners, and I am able to unlock my phone for now. And no cases like mine in the search results.

I'm interested, what could have triggered it? What data could have been compromised? I remind you that there were no messages about lost mode or erased iPhone, no scammer messages asking for money for the unlock, nothing like that. But still, it's pretty creepy. My email's and Apple ID's passwords are different and pretty secure, I have 2-factor auth on them too. Also, when Pokemon Go came out, I created second Apple ID to download, but surely didn't log in back to that account for weeks, just used my main one as always. And never had any other Apple IDs on my iPhone.

What a person should do to trigger what I experienced today? What data did he need to know? And can a free wi-fi network be a factor? And what about the time, as if it was scheduled? Or was it an iPhone glitch without any third party involved?

What can I do to secure my phone and accounts besides changing passwords?
Activation lock being on is a good thing; it means that your device cannot be activated on a different account or carrier until it's disabled, usually at the time you sell or otherwise dispose of your device.

I'm not sure what would cause it to randomly pop up like it did - the first (and really only) thing that comes to mind is your carrier making some adjustments on their end that caused your phone to need to be reactivated, but as you saw, no data is lost during this type of activation. It's also possible that something was corrupted on the device that required reactivation with the carrier to resolve. You would also see this screen if you put in a new SIM card.

You should change your password for your Apple ID to be on the safe side, but it's unlikely that any third-party, aside from your carrier, was involved in this incident.