It is unlikely that the temp sensor not being hooked up will cause the problem you describe. Ordinarily the sensors take the circuit to ground when the temp gets up there and thus the light/buzzer are activated. If the sensor is disconnected it cannot make the circuit to ground.
However . . . if the wire that is disconnected from the sensor is accidentally permanently shorted to ground (terminal stuck in a small space) then of course the circuit is made and the light/buzzer are activated.
Alternately, the wire is still connected and the sensor is simply faulty.
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