"Shipshape Home", a 1952 short story by Richard Matheson, also the answer to this old question; first published in Galaxy Science Fiction, July 1952, available at the Internet Archive.
The protagonist is a guy who works in an office building and notices one day that the janitor has an extra eyeball in the back of his head that is normally hidden by his hair.
The story is narrated by a guy, but it's his wife who notices that the janitor in their apartment house has a third eye:
"I walked past him. I felt myself shivering, because he looked at me as if he knew something about me I didn't even know. I don't care what you say, that's the feeling I got. And then . . ."
She shuddered. I took her hand.
"I felt him looking at me."
I'd felt that, too, when he had found us in the basement. I knew what she meant. You actually felt the guy looking at you.
"All right," I said, "I'll buy that."
"You won't buy this," she went on grimly. "When I turned around to look, he was walking away from me."
"Oh, you both turned at the same time?"
She slapped the table. "I turned. He didn't."
"But you just said—"
"He was looking at me. He was walking away and his head was to the front and he was looking at me."
I sat there numbly. I patted her hand without even knowing I was doing it.
"How, hon?" I heard myself asking her.
"There was an eye in the back of his head."
[. . . .]
"His hair parted, Rick, and before I ran away, I saw the hair going back over it. So you couldn't see it."
With some snooping, he figures out that the office building is actually a spaceship.
She pulled me and I guess I felt too ashamed of myself to pull back. WE started down. Then it came to me—if she was right about the doorway and the engines, she must be right about the janitor and he must really have . . .
I felt a little detached from reality. East 7th Street, I told myself again. An apartment house on East 7th Street. It's all real.
I couldn't quite convince myself.
We stopped at the bottom and just stared.
Engines, all right. Fantastic engines. And, as I looked at them, their structure, it occurred to me what kind of engines they were. I'd written some science articles and read a lot more.
One day, the building starts vibrating, and this guy knows he's got to get out. He hurries downstairs and runs outside. The ground beneath his feet continues to shake.
"It's true," Ruth said, horrified. "Oh, my God, it's all true then!"
I made a dash for the window.
The place began to vibrate, as it we were about to get hit by an earthquake. Dishes started to rattle and fall off their shelves. We heard a chair crash onto its side in the kitchen.
"What is it?" Marge cried.
Phil grabbed for her as she began to whimper. Ruth ran to me and we stood there, frozen, feeling the floor shake under our feet.
"The engines!" Ruth suddenly screamed. "They're going now!"
"They have to warm up!" was my wild guess. "We can still get out!"
Suddenly, he realizes that the spaceship encompasses the entire block. He sprints toward the street, but just as he reaches the sidewalk, a metal wall shoots up from the ground, basically encapsulating the entire block with him inside, and the block blasts off into outer space.
And then the sky, which was growing light, grew dark. My head snapped around. Women were screaming their lungs out in terror. I looked in all directions.
Solid walls were blotting out the sky.
"We can't get out," she said. "It's the whole block."
And then the rockets started.