A warning device (100, 212, 301, 302, 510) for use in a luminaire (201, 410, 501), comprising a housing (110, 210, 310) accommodating a drive module (115, 215, 401, 402, 520) arranged to be coupled to a luminaire and to a mains voltage (260, 560). The drive module comprises a light source (120, 420) coupled to a switch (180, 380, 491, 492) and an electrical interface (140, 240) configured to electrically connect the drive module and the mains voltage to a light element (530) of the luminaire. The housing comprises a lid (130, 230, 330) operably coupled to the switch of the drive module and configured to activate the switch upon an opening of the lid for switching on the light source upon a current of the mains voltage passing in the electrical interface and deactivate the switch module upon a closing of the lid for switching off the light source.