I've had a similar event in a Pa28R (totally my fault as I had forgotten to latch the door - a classic getthereitis case). What surprised me about your story FK73 is that the noise of an open door didn't alert you to the problem. The wind noise was so loud in my case I could barely hear the radio at 100 knots. I was alone at the time so elected to return to the airfield I had recently departed, land, close and latch it properly and depart again.