Hi. Welcome to Just Answer.
Although the outlet is controlled by a switch there is only one wire that actually goes to the switch. There are still two wires (neutral and ground) that go to the outlet that are not switched.
If there is another appliance attached to a different outlet that is back feeding the 24 volts via the neutral line then that is how it is getting there.
To determine what is doing this have someone watch the volt meter and begin unplugging things. When the voltage drops then the last appliance that was unpluged is the suspect.
Please let me know if you need anything else.