I've had this Guardian Angel Devices "Elite Series" LED safety strobe on my SAR pack for a year or so now. It's USB charged, has a fixed internal lithium ion battery, and is available in a range of configurations.
It's come in handy a lot on urban missions where we're searching along road sides... Hi vis gear doesn't work well when you have a backpack over it blocking the view of all the reflective stripes.
The thing is supposed to be waterproof but …