Originally Posted by Mr. Dent
It's Ultron. I swear people like to try and outthink themselves. It is Ultron, all the bystanders in the area say it's Ultron, the movie is called Age of Ultron. But if you want to think it's Dreadnought okay.
Its not called "outthinking". Its called "deductive reasoning". What do we know already? We know that the twins were in HYDRA's possession. We know that the fortress scene had Hawkeye, SW and QS. Its not too much of a stretch to say that the fortress is probably HYDRA. These scenes in question with the "Ultron" stand in also have Hawkeye, SW and QS in them. So its safe to assume that it takes place around the same time. Its also been reported that this sequence takes place earlier in the movie.
What is more likely, that Ultron is there in his final form with HYDRA, capable of decimating Hawkeye and the twins, or that its just a lighting reference for the goons in the mocap suits. But, of course bystanders with little to no comic book knowledge are always 100% accurate, so its gotta be Ultron.