I suspect visual alarm device. Often a red LED strobe light. Used in places like hospitals and old folks homes where hearing impairment is likely or in very noisy environments where an audible alarm may not be heard. 

Is it a VID or a VAD?
During a inspection last year I think,  a fire alarm system I installed was flagged because the Visual Indicating Devices (VIDs) needed to be replaced with Visual Alarm Devices (VADs). At the time, I was unaware of this requirement.

Visual Alarm Devices (VADs): These devices are designed to provide clear visual signals during emergencies, thereby facilitating evacuation. They are rigorously tested and certified to comply with the EN54-23 standard, which ensures a minimum level of light output and reliability. VADs are crucial in environments with high ambient noise levels, where individuals might use hearing protection, or for those who are deaf or hard of hearing.

Visual Indicating Devices (VIDs): These devices are intended for supplementary indication rather than primary alarm signaling. They do not meet the EN54-23 standard and are typically used to provide status indicators or to attract the attention of emergency services. VIDs should not be used as standalone alarm devices.

so we swapped the VIDs for VADs

- Andrew

Looks like a VAD to me, but noting Andrew's point it may be worth trying to get a part code to verify.

As an aside, is the exit sign meant to be non-maintained? Also doesn't look to have a charging indicator.