Mkii key and/or button?

Sorry.....similar to an annunciator panel or central warning as they are sometimes called. Any sensor that reaches a pre-determined threshold limit will trigger the warning or caution lamp. Similar to a low oil pressure warning lamp, but you add in temperature limits, EFI sensors, AFR or low-fuel pressure limit, and so on....
You lost me, Tom. A warning of what?
Of whatever was wrong!

A master caution light is tied in to a separate warning light panel. Any time a monitored system has an issue, such as brake fluid volume level, water or oil temperature, oil pressure, tire pressure, etc. etc. etc. it would illuminate a specific warning light and also the master caution light which is in the driver’s line of sight. He can then look over at the warning light panel to see which system is affected.