Hello,
Liquid damage could definitely cause the issue you are experiencing. In my experience, it isn't always a known incident that can cause that type of damage. Many things could have happened when you were away from your computer - someone else spilling liquid near the computer, pets relieving themselves, high humidity situations like steam from a bathroom or an extremely high dew point in the room, rain/snow when in a travel bag, etc. There really isn't any way to tell what happened.
It definitely isn't in Apple's interest or directives to report false evidence of liquid damages. There are sensors in the Mac that change from white to pink if there is any form of liquid introduced to the internal components. In addition, corrosion of the metals internally can happen quite quickly, and are a surefire sign of liquid being introduced. There aren't any commissions for the Genius Bar or store management employees for increasing the number of paid repairs. In my experience it is the opposite - Apple employees are trained to give some of the best customer service in the world, and regularly make exceptions for some damage situations to make customers happy and keep them for life. When they quote evidence of liquid damage, it is normally beyond a small bit, and has affected multiple components that can be costly to repair.
Did they give you any specifics with regards to the liquid damage, the components that were damaged, the cost level tier, etc.?
Hope that helps a little,
C
