It generally depends on state law.
In states where I've studied the laws, "plain sight" = "not concealed" and "concealed" = "not in plain sight". "Plain sight" is typically defined in plain English as "readily visible and identifiable to an observer". This is what most cops use as the rule of thumb for "plain sight" searches; if, by standing at some location the officer may legally be, and looking in some direction, they see the gun and clearly identify it as such, the gun is in "plain sight".
However, in your specific case you're treading on very thin ice. Most case law I've seen states that holsters constitute a form of concealment, if by a combination of the holster and any other concealment measures (intentional or otherwise), the object at your waist is no longer obviously a gun. If a police officer looking at you could say "well, it might be a gun, but it could just be a tool pouch or cell phone holder", and then finds a reason to Terry stop you and finds out that yes, it really is a gun, you could find yourself on trial for unlawful concealed carry.
The solution is simple; if your jurisdiction allows you, personally, to open carry but not to conceal, then make it (tastefully) obvious that what you have at your waist is indeed a firearm. If that requires tucking in the front right or back right of an otherwise-untucked shirt, so be it.