My Macbook air is asking for 6 digits pass code. But I didn't turn Lost mode on. What happens?

My macbook is asking for pin lock code (6 digits). Usually it happens when we turn Lost mode on and lock the device remotely. But I didn't lost either lock it. I just left it in the office for one night, on the next day when I opened the mac, it was asking for the 6 digits pass code. What to do next?

Context:

• I have 2 macbook air. A Mid 2012 and a new 2018 Macbook Air. The locked one is the 2018.
• I just bought the 2018 used a month ago at a local store, still has apple care until Jan.2020
• When I login to Apple I still see it in my Devices list
• But when I login to icloud.com/#find, I can't find it in my device list. On the other hand, I didn't turn Find my Mac on both of my mac.
• I never notice that in my device list, the new macbook air name is macbook's Macbook Air even though I already Login with my Apple ID. It's supposed to have my name on it like the old mac?
• When I purchased the new 2018 Macbook Air I noticed that it has another admin account in the system, I deleted it but somehow when I restart the macbook I can still see the other admin account while login. Does this mean I bought a stolen macbook and now it has been blocked remotely by the real owner or this is a bug?

Please let me know what should I do?

Thank you very much.