Might be time to open up the trunk foam blocks above the central locking module in the passenger side of the 'trunk'. If ever there was water down there, you might have corroded wires/connections causing this many problems. Since the siren sounds so robust (although it may still be the siren) you just might have a SKREEM module problem. If there is no damage at the central locking module, I would try unplugging the siren for a little while. If it stops, replace siren. If not, really check the central locking module. If that looks/operates/seems to be 100%, only thing I can think is the SKREEM module. Its going to be either easy, or expensive I fear!