This is to flag a possible PVR software bug in the parental lock/ratings settings triggered by a rollover from Dec 31, 2012 to Jan 1, 2013.
I was watching a recorded show on Dec 31, 2012 at 11:30 PM through to 12:30 AM Jan 1, 2013, an episode of a marathon of a series (entire Falling Skies first season) that we had recorded earlier on Dec 31, 2012.
We started playback of a recording at about 11:30pm Dec 31, 2012. At one point I had stopped playback at about 12:15 AM Jan 1, 2013 to take a short break. When we attempted to resume playback, all the episodes of this series had the padlock icon next to them and we were required to enter a PIN# to resume playback. We have never set a Parental Lock PIN# on our DCX3510-M, and our parental lock rating settings are set to the least aggressive, the shipped default settings. Only a reboot of the DCX3510-M removed the padlock icons and we could resume playback of the recording we had been watching.
I hope this helps anyone who may have encountered a similar problem, and that a reboot of the PVR apparently solves the problem.