That's right. There were no witnesses, but it's all about timing and duck placement. if you do the explosion too soon then the one of the guards from nearby comes through and finds the body. If you detonate too late then you don't get "No Unnoticed Kills" registering. Also if the duck is not placed in the correct centrally located spot then although all the targets are killed, one of the targets dies moments after the others and sees one of the bodies. So you lose No Bodies found bonus.
As I mentioned the most difficult part for me was the double coin drop lure. @LSxO, who first discovered this trick, has given me some advice on my YouTube channel, so I'll try that out. But basically you know if she's going to go for the second coin location if, when you turn around after dropping it for the second time, she is only just beginning to move towards it. If when you turn around she has already started moving then she only goes to the location of where you first dropped it, not the second location. I literally spend hours doing that first part, restarting again, and again, and again, getting it to work maybe 1 in every 20 attempts.