Interesting report on Avherald about a Dash 8 400 that took off from Sydney and flew half way to Albury before they realised they'd forgotten to put the gear up. Not having flown the 400, I don't know what indications they may have had, but I would have expected it to be a bit more noisy? And no doubt the increased fuel burn must have been a concern - I wonder if that's how they picked it up, or if someone in the cabin said something?
Incident: Sunstate DH8D at Sydney on Jul 12th 2021, crew did not retract landing gear