If the switch for the electric element, that's located on the HW heater itself, is on, then as soon as you plug in your 110VAC, it will start heating. Sounds like you're lucky that the HWH had water in it, or it would have fried the heating element. Maybe the dealer left it on from your PDI??
1988 Coleman Sequoia - popup (1987-2007)
2012 Roo19 - hybid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10