I’m with the group that says Black Widow jumped on The Hulk idea.
I can’t gif it, but after she talks about the Hulk, she walks to the side on her comlink. Loki’s following her both with his eyes and he’s actually walking across the floor.
But his face looks absolutely bewildered. He seems to have no idea what she’s actually talking about or how she got to that conclusion. He definitely takes advantage of the chaos later, but he had agents coming to get him for that purpose.
Now, he could be acting, but the unleashing the Hulk thing always bothered me. We don’t know how much Barton knew about The Hulk- or if Banner would be brought onto the project- but if the tesseract was to just drive a wedge, the Hulk would help but not be necessary.
In addition, he never mentions the Hulk. He figures the cage is for someone else (though how SHIELD would get The Hulk down there is another issue) and he refers to them bringing a monster on board, but he could easily be referring to, say, Thor. He may not know what Banner looks like or that Banner is The Hulk. (though he could have learned that last bit from Barton/Hawkeye)
Anyway, the fact that Loki physically follows Black Widow along with his expression makes me think Black Widow was projecting. I’m curious what his plan would have been though.