Is this a used system to you? It’s likely the previous owner has a liquid spill and the brown goo was collecting underneath the keys on top of the backlight plate and when you stowed your system the bottom became higher than the keys thus dripping onto them.
The only thing that has any liquid is the electrolytic within the battery cells. I would be surprised if that was the issue. You can check your batteries health using this gem of an App [https://www.coconut-flavour.com/coconutbattery/|CoconutBattery] post a snapshot here so we can see things as well.