Full Transformation's penalty always starts at -3
Compromise is a little abstract, just like Cover - a Demon that fails a Compromise roll means that the GM is closer on its trail, somehow.
For example, monitoring the web, it might come across a blog post where a woman is remarking she overhead her pastor having a (profane) conversation in Russian to an empty room. Or, an Angel might perform regular scrapings of a Cryptid fungus that changes color when exposed to Aether. Even though these are quite different, they might be the "how" the Compromising event makes it back to the GM.
Any Conditions that are gained from the result of this roll are applied after the demon reverts back to her human form.
DtD, page 195, Full Transformation
Since this passage implies that full transformation gets traced to the Cover the Demon resumes, it indicates that the Compromise is less "Then my girlfriend turned into this screaming pillar of blood and glass, then flew over my head! #yolo" on twitter, and more akin to preternatural ripples that even Demons can't detect - but there is Infrastructure that can.
From a game balance perspective, Aether is meant to be a powerful fuel, offset by the risk in acquiring it. If players could just have their Demons "sneak around the corner and vogue a couple times" to refill their Aether pools with little recourse, it would certainly shift the game's tone.
This lends credence to the idea that full transformation is at a steep penalty because it is such a reliable way to gain Aether, and any leniency for seclusion's sake wasn't overlooked - it was deliberately not included.
Bonus (if a little cynical):
From a strict reading of the rules, it never says anything about witnesses in relation to full transformation; ergo, it always starts at a -3, regardless of witnesses or not.
Now, if there are witnesses, that's a further -2 (Demonic forms are "overtly supernatural"). But if the witnesses are unreliable, that's a +1 for a net of -4.