Knox
I'll share my experience for what it's worth.. I returned my S4's and picked up two S5's. The 4's had been heavily mod'd and were, of course, rooted.
While the lady at the store was getting my new phones ready I did a factory restet/wipe on the spot. It took a minute and she just glanced at it. I powered down, popped my SIM and external SD and handed it over. Done.
The Knox counter had been tripped and the status for the phone was 'custom', not 'official ' when it's stock.
Maybe I got lucky with my rep but she didn't seem equipped or interested to even check the device and by the time it gets back to Samsung for reconditioning it won't matter.
Granted, this wasn't a service call or a repair issue with a trained monkey. If they're doing their job, they'd quickly spot the flag but even so, most don't care in my experience.
Sent from my SM-G900W8 using xda app-developers app