What marketing department on earth is going to pay for an ad campaign so subtle that they can't even tell if it's running?

Ad budgets aren't bottomless. People making decisions about them have a lot of options for where to spend. They want provable attribution so they can tell which channel is giving them the most bang for the buck. If that exists, then the ads will be discernible.

I‘d argue that, at some point, this behavior will surface enough that it gets interesting for government to take a look at it. At least in my country, ads need to be labeled as such - I don’t think this is going to change.