Someone in the original thread had a link to another forum, and the gist of it was that the russians had been engaged in wargames, a month before they actually went in.
As for going un-noticed, that’s the perfect outcome for any military and the russians are masters of the macro rope a dope.
The intel guys can weigh in with real world stuff, but think about the mechanics for a sec. Assemble your force structure, so now you have tank regiments mating up with low carriers and rail stock, that can be tracked by recon and signint assets. Add infantry and aerial assets that conduct an increase in live fire exercises and realistic training.
In popular fiction some one jumps the chain of command after getting reamed by the boss, and saves the day.
In real life, you know someone is going to have a bad day, but its only going to be 20/20 hindsight that you should have seen it coming.