What does the invisible guest mean by missing the pen?

This pen is not an ordinary pen. It's actually a recording pen. With the help of this pen, the film laid the groundwork for finding the murderer.

The female lawyer also deliberately left the pen to the man. At the last moment when the time was ripe, the man's shirt was stained by the water leaked from the pen, which means that the man is the murderer.

abstract

Adrian (mario casas) is an entrepreneur with a thriving career. He has a beautiful and virtuous wife and a lively and lovely daughter at home. Because of his fruitful career and family, he is the envy of others. However, the ambitious Adrian did not cherish the hard-won life in front of him. He has been keeping a physical relationship with a female photographer named Laura (Bárbara Lennie).

After a tryst one day, they left the villa by car, but there was an accident on the way. In order to cover up the truth of the incident, they decided to sink Daniel, a young man who died in a car accident, into the lake with his car. Later, Laura meets a kind old man (played by José Coronado) who takes Laura's broken car home to be repaired. What Laura didn't expect, however, was that the old man was Daniel's father? .