The God Machine corebook implies that in the absence of Concealment Infrastructure, the gears and cogs of the God-Machine are visible. Concealment Infrastructure has two main effects - mundane camouflage such as a storefront or warehouse shell, and a supernatural cloaking effect which renders the gears invisible to mortals. The two-dot merit Unseen Sense: God-Machine will allow a mortal to see through the supernatural cloaking effect of Concealment Infrastructure, as well as giving them a general sense of the presence of the God-Machine and its agents.
If the blood treatment plant is concealed, the players could discover it by one of them already having the Unseen Sense: God Machine merit or by accidentally or deliberately bypassing the Concealment Infrastructure. It may also not have the supernatural cloaking effect, because it relies solely on mundane camouflage or because the Concealment Infrastructure has been damaged or disabled by (for example) glitches, demonic sabotage, insufficient maintenance or the God-Machine itself as part of a larger plan.
Exposure to the God-Machine can cause humans to become stigmatic and thus gain the Unseen Sense: God-Machine merit, as well as other effects. The "Becoming Stigmatic" section in the Demon: the Descent corebook does not give mechanics for the initial transformation, leaving it up to the ST. If you find it necessary to include specific mechanics for becoming Stigmatic, there is an example that can be generalised from in the section titled "Madness Under Glass", where a mortal viewing the artwork "Heaven" makes a Resolve+Composure roll - 2, and those who succeed become Stigmatic and those who fail gain the Spooked Condition.