Didn't see it myself but my brother in law was visiting Phoenix around that time for work and mentioned something odd in the sky on Thursday evening, said it was dead silent which is the bit that got my attention. He's not really into any of this stuff so he wouldn't have known what to look for or how to describe it properly.
Can I ask - when you say triangular, are we talking a tight equilateral shape or more like a broad flattened triangle? And was it moving at a steady pace or doing that kind of stop-start thing some of these objects apparently do? I've been reading about the Belgium triangle wave from the 90s and trying to work out if theres a consistent shape pattern across sightings or if people are just pattern-matching onto the nearest geometric shape they know.
Trying to figure out if its worth ringing my brother in law and getting a proper account from him before he forgets the details.