Hi guys, same issue. I repair computers at cameras in palmetto. Bay Florida and, the owner had just bought to 27” imacs but, had them reset but forgot the password on both. He is a relatively wealthy guy and, ex police officer but, he is on the older side and usually keeps his passwords on an older pda. At any rate, he lost his little memo taker and handed me the imacs. I was able to quickly get one ready and upgraded to high sierra but, the other one was apparently handled by a jealous tech and, the firmware password was apparently set. I say apparently because after trying all standard solutions from keyboard commands to Ram and battery removal, I have not been able to reach anything but the dark gray lock so, I'm beginning to suspect third party software but, I just don't know enough about newer imacs to be sure.
I just read a post that suggests the utilities menu comes up after a while after getting to the grey lock and, I will try that at the shop tomorrow but, I surely would appreciate some guidance!
Best to you and, may you all have a wonderful weekend!