Didn't see it myself but I'm up in Yorkshire so not exactly local. Curious about a couple of things though - was the triangle holding a fixed formation or rotating? And did it make any sound at all? I ask because the Lake Michigan area has had a few decent sightings over the years and the silent low-altitude triangle is a pattern that keeps coming up in that region specifically.
Also worth knowing - what time exactly and what was the cloud cover like? Reason I ask is that thermal inversion layers over large bodies of water can do some genuinely weird things to light from distant aircraft, but that usually produces smearing rather than defined shapes. If you're describing a sharp triangular outline that held its geometry while moving, that's a different conversation entirely.
Any photos or video even if its blurry would help a lot here.