I am a Spanish passport holder. I am living in the US, in the process of getting a Green Card, and have work permit issued by the US Embassy in Spain. The work permit is expired, but I am still in the US. I am travelling to Seattle from Austin, TX. Does the Seattle Airport have CBP where I would have to show my passport (it is technically within 100 mile border zone)?
US embassies do not issue work permits. They issue visas. The visa does not confer any status; it only allows you to apply for the indicated status at the US border. Whether the status remains valid while you are in the US is determined by the I-94 form.
Additionally, most nonimmigrants who are authorized to work in the US do not have a work permit, but rather their nonimmigrant status carries with it the implicit authorization to work.
If you have already filed for adjustment of status (AOS), however, the I-94 may not be particularly relevant. In that case, your work authorization would depend on a work permit, called an employment authorization document (EAD) issued (in the US) by US Citizenship and Immigration Services (not by an embassy or consulate).
On the subject of the much-misunderstood 100-mile border zone, it does not have a huge legal bearing on whether you could be asked to show proof of your immigration status. Any officer who is authorized to enforce US immigration law can question you about your immigration status if they have probable cause to believe that you are out of status, no matter where you are. But, as a practical matter, you are more likely to encounter an officer of the US Border Patrol when you are within 100 miles of the border. Whether they routinely operate in the Seattle airport, I do not know.
What does this mean for your question?
You will not encounter CBP field officers as a matter of routine, but there may be a slightly higher risk of encountering US Border Patrol in Seattle than in an airport more than 100 miles from the border.
If you do, you should not worry as long as you are in status or have a timely application for adjustment of status pending. Having a valid visa is unnecessary.
You are not required by law to show your passport to immigration officials if you have another suitable form of identification. There is a law that requires you to show any document you may have been issued from a certain list, but this law is not frequently enforced. It is more commonly misused as a pretext to require aliens to carry their immigration documents with them. If you have any immigration documents you've been issued by the US government, you should be fine. (Of course, a passport with a valid admission stamp is evidence of lawful status in the US, but it is not definitive evidence, and other documents such as your I-94 or AOS receipt are better evidence of that.)