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In all modern day vehicles there will be some degree of current draw for 'X' minutes after turning the engine off and locking the vehicle. This is due to the electronic modules which will still be running and communicating with each other. The modules all shut down at different rates and eventually enter into a sleep mode until such time that you unlock the vehicle again.
What the current draw should be on your vehicle while this is happening I couldn't tell you. This information maybe available via your local dealer.
Hope this helps
It shouldn't go flat in 3 weeks however the question you asked was if 3.5 Amp current draw was normal. The answer to that is yes if the modules haven't all gone into sleep mode at the time you were testing them.
I can't talk for Ssangyong however on a Nissan within an hour all of the modules should be asleep after locking the car. This in turn will make the current draw approx 0.1 amp if you allow for any alarm systems that have to have constant power to monitor the vehicle.
If this has only happened since you had these what I assume are aftermarket parking sensors fitted it is entirely possible that when they were fitted the power for this was taken from somewhere that is now preventing a vehicle module from shutting down.The only way to check this would be to disconnect the parking sensor module that has been fitted and see if that cures it.
When fitting accessories the only safe way to ensure that they won't cause an issue to the vehicle is by ensuring that they are approved by the Vehicle Manufacturer.