Normally, one would associate flashing lights with vehicular warnings but I've seen them used as an outdoor warning as well. Before the Chino CIM Prison installed their three sirens, their main warning device were blue strobe lights on the prison's water tower and at the (now decomissioned) Aerojet Ordinance Facility in the hill behind my neighborhood. The blue strobes are still used in conjunction with the siren system today. I noticed that the two siren sites I've seen on the prison property also had smaller blue strobe s located near the siren which began flashing three seconds before the siren activates.
There are also two places in my town where I've seen a red strobe and a plain light bulb with a red cover were used as a warning device. They were affixed to the top of some electrical cabinet and there was a sign nearby that said to call a certain phone number if the light was flashing.