I get a quick flash message saying "security feature not available" or some terminology every time I let off the brakes.
It flashes so quickly I can't catch it clearly. It goes away as quickly as it comes. Nothing new with or added to vehicle.
It's a 2015 Cherokee Limited V-6
Any insight?

 

Scan it for trouble codes. It should be stored.

 

Never heard of "Security Feature Not Available". I know with the blind spot and other things, you have to keep the sensors clean in order to have them work correctly. The full 2015 Cherokee owner's manual is available online on Jeep's website (FCA/Stellantis is just TOO cheap to give you a full manual anymore, and only gives you a small "User's Guide"... my 2014 Grand Cherokee WK2 is the same way and it's annoying sometimes). The full manual should give descriptions of every warning message the Electronic Vehicle Information Center (EVIC) can give. There probably isn't a DTC trouble code, but just a message. Sometimes you can go through the EVIC using the steering wheel directional buttons/"D-Pad" on the left side of the steering wheel to see 'Stored Messages'. That may allow you to read the message again.

-Vince