If K-2SO is alone, then who is "we" here?

Question

On planet Eadu, Captain Andor and his team went to search Galen Erso and leaving K-2SO alone in spoiled Imperial plane:

K-2SO: What are they doing? If Cassian comes back, we're leaving without them.

If K-2SO is alone, then who is "we" here?

Answer 1

He is referring to himself and Cassian as 'we'. The 'them' that he is referring to are Jyn, Baze and Chirrut.

Note that he's explicitly confirmed to be talking to himself in the official novelisation.

The tragedy of K-2SO’s existence was this: The skills he most cherished were skills his rebel masters disdained; and the skills he considered crude and trivial were skills his masters were helpless to learn.

Thus, his present circumstances: Instead of traveling to the research lab to manhandle, capture, restrain, and extract the scientist Galen Erso—a mission virtually requiring the talents of an Imperial security droid, and which might (if handled delicately) permit the exercise of multiple underutilized procedures hardcoded into K-2SO—he was rewiring a communications array and locating faults in each of eighty-four connectors by touch.

Such a task required a bare minimum of computational power. K-2SO had more than enough to spare to listen to the goings-on in the cabin and observe the landscape from the half-buried cockpit viewport.

He watched Jyn depart with disinterest. The woman had always verged on disrespectful toward him.

He watched Baze and Chirrut depart with more robust disapproval. He posited an array of scenarios involving their separation from the U-wing, few of which ended in their continued well-being.

“What are they doing?” he asked sharply.

K-2SO was not a protocol droid, but he was designed for biological interaction. He found that verbal discussion, even with himself, spurred his creativity.

He soon came to a solution he was satisfied with.

“If Cassian comes back,” he said, “we’re leaving without them.”