There are a few reasons why you're hearing the mixer left on for a while, prime of which is the new setup. The powers that be moved the ON AIR lights. We used to sit in a rough circle, everyone facing the center of the room. The lights were on top of the radio console. If you left the mixer on, it was apparent.
Now we sit with our backs to each other facing out and the lights are behind all of us. No one sees them unless they turn around. (And, since we no longer can see each other, nonverbal communications [nods of the head] are out. This forces us to yell at each other thus the increased noise you hear in the background.)
The radio is no longer a console, it's a touchscreen LCD. It's mounted so far away that one has to use a mouse. The icon for the mixer is only a few pixels larger than the cursor. It's very easy to simply miss hitting the icon. This also has the effect of delaying our response time on the radio, one has to move and aim the cursor before clicking.
Also, believe it or not, we have a few people in our employ that do not own PCs and have a hard time using a mouse.