why is the key showing up at the top of screen

My phone is on lock at top of screen

What device do you have? Do you have a VPN set up? Are you on WiFi? Which side of the notification bar is the key on?

If you have a Goole-branded device (Pixel, Nexus) and you're on WiFi, this could be the WiFi assistant automatically securing an open WiFi connection.
https://support.google.com/nexus/answer/6327199?hl=en I believe this now applies to non-Fi phones as well.

If you have a VPN set up and it's connected, this is an indication of that as well.

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