FR likes to make cabinet doors that don't open. Instead of a knob, there's a third hinge on the knob side. See left cabinet door in the pic below. They like to put things like HWH valves, slider controllers, etc. behind these. If you have one of these, take the screws out of the 3rd hinge and see what's behind it. Even if it's nowhere near your water heater, I'll bet there's something behind that door that you ought to know about.
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