Safety Conscious

I have a Ventilux emergency light in the corridor of my apartment. For the past few weeks, it has been staying on most of the time, especially at night. It is 'Self-Contained without Rest Mode'. I don't know why, but is it likely to remain on if the battery is depleted or is there a battery in this unit. I have not done anything to it since I moved into the apartment in 2005. Thanks.