Can I just say that I always found the notion of Sentinel robots stupid? I mean, it's visually cool to have these giant humanoid forms flying around threatening mutants, but in a world of superheroes, it's kind of dumb.
Suppose for a moment that the giant robots are from a government organization, convinced that mutants, a subset of superhumans, are a threat. The information about where they are housed is part of a bureaucracy. Even if it's secret, someone has to know. Someone makes the parts, someone buys the parts, someone ships the parts, someone assembles the parts. Someone knows about it. Someone will leak it. Can it be hidden? Yes, but you need something on the scale of, oh, North Korea to hide it.
Because these are big frickin' robots. You can see them flying around, and I haven't seen any indication that they're supersonic.
Okay, suppose the organization isn't funded by the government, but is instead a secret organization?
Then it's worse. Why doesn't some other superhuman track them to the airbase or headquarters? The superhuman doesn't even have to engage...he/she/it just has to identify the location and others can lead the raid. I mean, this are some seriously big robots we're talking about...they hold a dozen mutants in their bodies and still fly.
In our world, it's hard to hide an airbase. In a superhero world, that much harder. If the base hasn't been found, then that means that no superhero has put her/his/its mind to the task.
In the Strange City world, I suddenly realized that one of the ways that the next series of VIGILANT robots stay hidden is that they aren't big-ass giant robots at all. They're human-sized (or slightly bigger than human) subduing robots; the big-ass form is when they combine to carry targets. There are sniper robots, fighter robots, whatever, and they combine together if the target is to be brought to an airfield. From the airfield, the captured target is then transported in some other way...by rail, or in one of seven identical black Escalades, or something.
The point is, the big mega-robot exists only when needed. It's too noticeable, too easy to see, for other purposes.
You lose some of the visual oomph of the giant robot, yes, but you gain a little in plausibility (even if robots that can combine is just as implausible, it's slightly more resistant to the normal player counterattacks).