Patient confidentiality must be honored, and leaving a computer open even to run to the bathroom for a minute risks a serious breach of patient confidentiality. Even though you are the only person in the room, you should not have access to patient information that you have no need to know. Even if you wouldn't say a word to anyone and you are a professional nurse with integrity, you should not be able to see possible protected information on a patient that you are not personally taking care of. Leaving a computer wide open is a serious risk to confidentiality.
If you see this, you could gently close the computer so that the screen is not visible and discuss the seriousness of this when your coworker returns. You must protect your license and you should report to supervision. It's uncomfortable to do so, but a nurse always takes the action that protects and advocates for the patient.