I stumbled over this message when I was searching as I had the same problem:
1. My MacBook Air was lost/stolen and I locked it with a pin from my iPad.
2. After more than a week I got the message that the MacBook was locked. Unfortunately you get not information on where the Mac has been online.
3. After another week the MacBook showed up, someone returned it.
4. I tried to unlock and got the message "This Mac has been locked, Try again in 60, 59, 58 minutes". After this I got the message "Wring Passcode. Try again".
5. But, there is no field to type in the passcode.
6. Finally the solution was: Open findmyiphone on the iPad. select the MacBook. It says "locked... will play sound when online". On the right hand corner of the screen, there is a command to click on "remove". Hitting this the Mac is removed from the list and removed from the "lost"-status. Immediately after this the fields for entering the passcode showed up on the Mac. I assume it is also important, that your found mac is on a location where it remembers the net code to go online. Its hidden, there is no sign that the mac went online.
7. after entering the right passcode the mac reboots and looks like has never been away or abused...
8. I also asked apple-support. they send me an information, but the steps described there lead to the same solution but the screen and commands there where different. So maybe an old version of the helpfile.
Hope this helps fellow users. Robert.