Isn’t it most likely leakage from the stanchions bases.I had the same problem on my 325 after 8 years of use. Stanchion bases get strained when crew climb on and off the yacht putting their body weight on the guard rails and eventually the bedding sealant cracks. I simply removed the stanchions and thru-deck fittings, cleaned out old sealant and re-sealed with Sikaflex. Has lasted another 7 years so far.
Leak below binacle – I had the same problem. Water can collect under my binnacle housing and made its way down through the sealant into the rear cabin – once again a Sikaflex job sorted it out. Now a full cockpit tent keeps binnacle dry year round.
Good luck
Allister
The membership association for owners of Dufour Yachts