Unless I'm wrong, the OP is not talking about the red key and red outlet with the red cap on it. That clearly looks like a battery disconnect unless somebody installed it for some other use.
What I believe the OP is referring to is the BLACK SWITCH in the BLACK HOUSING. Could be anything but these are commonly water heater switches, but those typically have the no-light-fault LED light next to them so you know the water heater did not light. In that case, it could be anything.
The OP can verify that if he finds another switch for the water heater, normally mounted higher than the one shown. In that case, could be anything.