Has anyone ever heard of someone leaving a few days before their DEROS or taking leave right around their DEROS during the holiday season?

Sometimes people leave a few days before their DEROS, it depends on flight availability. DEROS is usually the no-later-than date the servicemembers has to leave the country, so if they schedule a departure a few days sooner, it can ensure Soldiers don't exceed the timeline.

I had a fellow senior NCO that out processed and was done and showed up at Kimpo and got on a flight and left about a week early so he could be home for the holidays

In the 1980s there was a policy that if your DEROS was on a Monday, the Battalion commander at his discretion could allow a 4 day drop. To my knowledge our BC never disapprove any, including mine. I was able to leave on Friday instead of Monday. That was the only instance that I knew of perhaps others took it as well. My DEROS was in February.