And the next paragraph:

The jet had been prevented from making long-haul flights over water so that the plane “could return very quickly to an airport” in the event the warnings happened again, NTSB chief Jennifer Homendy said.

Which makes it sound like they couldn’t find the source of that warning but weren’t willing to completely write it off.

Nevermind:

“An additional maintenance look” was requested but “not completed” before the incident, Ms Homendy said.