One key has changed function

Using it in the bathroom !?!?! Thats not the best place to use your system.

Sadly this is not an easy fix what happened here is the soapy water got into the keyboard onto the switching elements damaging them. You will need to replace the keyboard to do this right. This is a very big job as the entire system needs to come apart. Here is the IFIXIT guide to gain access to the uppercase: MacBook Pro 13" Retina Display Late 2012 Upper Case Replacement. At which point you could take the keyboard out of the upper case and try to clean it or replace just the keyboard, or just replace the upper case.

As to how the keyboard works it uses a cross bar system (X & Y grid) so if you damage the contact board you can cause mis-keys as you encountered. Soapy water is corrosive which is why it got damaged.