Secret of the Painting

Many men would have killed to possess the painting—for Tom Calhoun knew it held a key to knowledge that would rock the scientific world!
Hold it, buddy, this fellow said, coming along the bar toward me. I want to talk to you.
The way he spoke set my teeth on edge. There was a whining, placating tone in his voice, but under this was a growl which indicated that if he had the chance, he would be glad to order me to stop and talk to him, instead of asking me. His clothes were expensive, but unpressed, and he was wearing them in a way that I didn't like. There was another thing about him that I liked even less—the slight bulge under his left armpit.
All in all there was only one thing that I liked about him—the way his lower jaw stuck out ahead of the rest of his face. It was a perfect target for a left jab.
You're Luke Shaw, ain't you? he asked.
I am. And so what? He looked me over carefully after I spoke. A faint flicker of grudging respect appeared on his face as his gaze crossed my shoulders. He measured me for a hidden gun, which he didn't find because I wasn't wearing it. He liked this. It made his job safer, if not easier.
Look, Luke, I'm not trying to stir up any trouble. The whining tone was back in his voice. I just wanted to know—you work for Tom Calhoun, eh?
I felt my back hair begin to raise as he mentioned Tom's name. So far as I knew—and it was my business to know—Tom Calhoun didn't have an enemy on Earth. He had me on his payroll for two reasons, the first being that I was the best friend he ever had, with the possible exception of Ann Briscoe, his laboratory assistant, the second reason being that he knew he could trust me right down to his last chip. Sometimes it gets important to have one guy you can really trust. My job was to shoo away all curiosity seekers, who would invade his lab by the scores just to get a glimpse of the great scientist, thus making certain that Tom got all the privacy he wanted, which was about all there was of this article. Also if the commies should come prying around, I was supposed to meet them and roll out the carpet edged in black. They had and I had.