Why does Adrián Doria stare at the pen after dismantling it?

Question

In the Spanish mystery thriller Contratiempo/The Invisible Guest, I do not understand the scene which occurs at the end of the movie. Adrián Doria dismantles the pen and with a surprise stares at it. Why?

Answer 1

Remember that he had problems using his phone throughout the film? Well, the pen actually had a device that disrupts the phone signal. From Wikipedia[*]:

Virginia leaves the apartment. At that moment, Adrian realizes the pen she gave him is actually a mobile phone distortion device, which makes the conversation become difficult to understand, (that's how Virginia manages to not get discovered when Adrian passes the phone to talk with lawyer Felix...)

The plot is very convoluted, to be honest, but if I recall correctly, Fake Virginia's entire plan hinged on his other lawyer, as well as the real Virginia, not calling Adrian and ruining everything. The pen took care of that.

[*] [I actually remembered that she used the pen to record Adrian's confession, but I'll believe Wiki, she probably used something else.]

Answer 2

ref: Contratiempo Explained

I'd like to add that, though the "bug" in the pen was distorting the conversation, a couple of minutes before this Adrián gets a message from his lawyer:

"Adrián, are you there? Your phone's off. Call me when you can"
Félix has been trying to call (but Adrián's is off) and finally gets through to Adrián with a message saying that he has removed the threat of the driver (the miracle they were hoping for).

When we are shown Adrián taking out his phone we are shown that he's restarting it, the apple logo appears. After this he even tells his lawyer:

"My phone went off. What is it?"

Towards the end we are shown a sequence where Ms.Goodman (Tomas' wife) switching off his phone and handing it back. So his phone is off, that's why he can't be reached. Not the scrambling (as opposed to wikipedia).

Now to answer the question:
The pen has a bug that is transmitting the conversations across to Tomas who is recording. The bug interferes with the signal on the phone, the interference comes from the pen. Adrián opens the pen to find the transmitter. The reason Adrián is surprised is because he thinks he has been talking to the real Ms. Goodman. When he sees no notes and finds a bug in the pen, he realizes that the conversation was being secretly transmitted. He just confessed to "everything" he has done and now he finds out that the confessions have been transmitted to someplace.

He looks into the notes that Virginia (Elvira) has been taking through their conversation and realizes that they are empty. As he looks through his window, he can see Virginia (Elvira) in Tomás’ room. She takes off her makeup to reveal that she is Elvira, the boy’s mother, and not Virginia. Adrián realizes the entire session was a setup and he’s given a detailed confession about everything.

The surprise on Adrián's face is the beginning of him realizing that he's screwed!