My Asus zenphone 3 Ultra keeps flashing

[AC Question]

Hi all,

I bought my Asus Zenphone 3 Ultra in Hong Kong about three months ago. About twice a day at random times, the screen starts to flash red and blue. As well it vibrates and sounds the alarm. No log or message comes up. I'm using 7.0.

Any ideas? I've already done factory reset and no joy.

Cheers,

Tim