Looking at the FC WFS plot here: summary pages SQZ subsystems you can see that the squeezer whistles have reappeared.  This is a possible explanation for the nonstationarity and poor range of DARM overnight (There's an issue with the summary page of last night's range right now, but it should appear here: range summary.  However, looking at the previous days, the squeezer whistles appeared on the 25th and have been getting more frequent, but we didn't see poor range until last night. 

As Ryan Crouch noted, last night was windy, but the wind died down around the time the IFO lost lock, and the last 6 hours have been calm with poor range.