[Milhouse]

It does sound like spurious reporting of under-voltage rather than an actual under-voltage condition.
Also if you see the symbol again, can you report output of "vcgencmd get_throttled"

I've only seen the lightning bold once or twice on a RPi3 with genuine white RPi Foundation power supply. The only times I've seen it is during the first second or two of booting - once it disappears it never appears again. I've only started seeing it in the last week-two weeks, never before with this PSU. It's hard to reproduce so I can't say at the moment if it's fixed or not.

If I see it again I'll run vcgencmd.