The cause might be an inductive noise emitted from a high-voltage test lead.
The withstanding voltage testers of TOS series light on the DANGER LED when they detect approx. 30 VAC at the output terminals.
The DANGER LED light alerts you of high voltage present.
The inductive noise can enter through the long test probes or high impedance test probes.
In addition, if an AC input power supply for the tester is not grounded, the DANGER LED also lights on.