I had several fleets fail to continue trade routes after danger halt expired. Fleets were at 0 danger and were still halted.

Were they attacked? If they are attacked, they do not start again until you manually start them. Which is a GOOD thing, otherwise HNs would be really dangerous.

Didn't know that.

Then you have simply not been attacked enough, I guess! ;-)

I've been attacked, just nothing recently on my trade fleets, I just don't remember having to reactivate my trade fleets from the danger halt.