So I just noticed that if I click on the 'conn' monitor, and then
the HISTORY button, all my hosts are showing red on that histogram.
Yet everything is fine otherwise. They aren't actually alerting,
just the history page.
I think you need to show us the image you're seeing.
The histograms consist of the time-markers (showing the hours, or
days, or months) and the color-bar. The time-markers can very well be
red without there being a red stripe in the color-bar; that is just an
indication that there was an outage e.g. in August, but it was too
short to show up as a stripe on the color-bar. (That's what the
"Min. duration shown" tells you).