I absolutely agree with your larger point towards the bottom, and goes along with it feeling weird for me in 4E when a goblin and a dragon could both become CR 15 challenges for no other reason than to actually make a threatening creature actually a mechanical threat should it devolve into violence.
In 3e & 4e you could 'level up' a monster, sure (in 3e, by literally giving it class levels), but it wasn't like you had to, there were plenty of monsters at any given level, and nothing illegal about allowing a lower-level creature to exist in the party's presence.
In 5e, with BA, it's not really an issue. A low-challenge patrol might be a minor encounter in a vacuum, but even if they're not equipped with magical body cams, the point of the patrol is the organization, not the patrol itself, and there's no way to entirely suppress information.
If sending scrolls fit the tennor and background of your take on the setting, that's just fine, you get a sense of verisimilitude with consumable items filling the role of technology, making the world pervasively magical, like the feel of Ebberron writ small. Or the magical solution might be more centralized, a tame, but very capable, Diviner who investigates missing patrols or the like (to keep magic more rare/powerful in the setting). Or the reporting or investigation could be more mundane. Witnesses could be questioned in an altercation happened in town, or patrols could be shadowed by informants, or have those homing pigeon equivalents someone mentioned.
Personally, my feeling is that heroes being harassed under color of authority is fairly meh, and would rather have a powers-that-be with which they could share some level of common interest or mutual respect.