People are on their best behavior for interviews.
Many people are great for a period of time after they're hired but certain people will always morph into their natural ways unless their bosses can somehow steer them in another direction.
Think about coming out of college from having no money into getting paid over a million per year.
I've seen guys just out of high school get a job for a little over minimum wage and they suddenly thought they were king of the world.
There were some questions marks in scouting reports about Taco's maturity but no true red flags.
The Cowboys likely waited too long to really discipline him. He should have bee inactive the very first time the coaches were not satisfied with his attitude/work-ethic.
Doing some google searches, there were harsh comments from Marinelli back in 2017 in the OTAs/training-camp time frame with regards to Taco with Marinelli saying "It's not good enough" when asked about Taco's progress. That seems more harsh than the normally silly antidotes that we hear from Marinelli when asked those types of questions.
Good response and, yes, people are more often than not on their very best behavior during an interview.
Aside from the interview, though, there must be multiple contacts with Taco's former coaches, friends, etc - the sort of background checks you would expect for a first round draft or any other high value employee prospect.
In my prior life (before retirement) we would, among other things, check Twitter, Facebook and other social media platforms to get a sense of the employee.
I guess my take is that there were most likely tell-tale signs that things may be off-center with Taco.