Regarding someone showing the knife vs hiding it...
Generally when someone shows a knife, the knife is being displayed for the purpose of "violence of action." In other words the criminal is displaying it to gain compliance. Often this is an economic crime. The best way to deescalate this is through compliance. They want your car, phone, wallet, whatever.... Give it to them. The best way to escalate this is to resist, now you end up getting cut, or killed by someone who didn't really intend on doing that. This is what the Krav Maga crowd hasn't seed to figure out....
Regarding the hidden knife. You don't have to see the knife to prepare for the reality that you are in a knife fight. Awareness of hand accountability is important. If you can't see both of his hands, it's a high probability a weapon is going to come into play. You can prepare mentally and tactically for a knife attack, prior to seeing it. If someone is not showing you both hands, begin to prepare that a weapon is likely about to be used.