981 / 991 suffered from a blocked grommet in the floor pan which stopped water draining out .. there was a recall on it to replace them .
For a 987 grommet fault then the car would only get wet when driven , the water amount wouldn,t be that great but im afraid off hand i dont know with out removeing undertrays and looking if theres a suitable grommet that could cause this .
I can say ive not come across it so far though .
If door seals are 100 % ok then i would move to the rear window seal but as said above you need to strip panels and water test .. which to be honest is what we do with any water leak , if no leaks found from above ( normal water ingress ) then we move to below .
Water leaks can often be far removed from where the actual water leak is found .. it can be a pain to find them .
A garage might be best to locate this fault i feel
Many good indys in your area , i should know .. i work at one of them lol .
EDIT ..
Its not actually the ECU or DME , its the rear CU and park assist cu there .. rear CU is part of the alarm system , if it gets wet then rear lights stay on , spoiler goes up alarm faults etc etc
Boreing fact of no real interest really .. sorry .