My 01 v70 would unlock front pass door would unlock in a turn! On one of the forums I read about micro switches and the need to clean them. I removed the door panel and sprayed everything with WD-40 wire connections, linkage, etc.
Working the door latch 20-30 times seemed to clean the switch.

You may want also to check the condition of the memory battery housed in the siren. You remove the wheel well plastic shield on the right side and fine it located in a bracket. It appears the battery lasts about 7-8 yrs. then leaks and destroys the circut board. If you catch it early you may be able to replace the battery. see Matthews volvo site for the topic and instructions. You could unplug the siren( about $175.00 for the part @ discounter) and see if the lock problem goes away!
Good Luck

I have had the very same experience with my 02 S80. Only when my wife was in the passanger seat. The passanger seat door lock is directly under the inside door handle. When exiting the car the door lock is flicked at times and the computer gets out of sequence whick results in the driver door locks and then unlock. After I pointed this out to people rideing the problem went away. Hope this helps

it's the actuator.
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Jake