Did you use any water during that period. Did the grey tank show a rise in water level. I have heard on some models (not sure which) the bathroom sink was plumbed into the black tank. Otherwise it should not be possible. It's like your fresh water tank filling your neighbors black tank. You may need to supply a little more information.
2015 RAM 3500 SRW, 4X4, 3.73, 6.4 Hemi
2016 Eagle 338rets, Maxxis Tire Upgrade, Slide Toppers, Backup Camera, 2 - 15K AC units
Blue Ox Sway Pro Hitch, 2000 lb. Bars