Honestly, everything else about the house is very well done so it does seem odd that they would get that sloppy here. There is some rubble and a bit of moisture in the photo. It's possible that rodents dug the rubble out but the moisture is from a leaking hot water heater (this has distracted my DIY energy over the Christmas break [I actually made a hilarious "lazy river" with vapor barrier that flows into a bin and an auto-siphon to drain it outside so we could have showers and dishwasher over the holiday]. All to say that moisture is coming from a know source.