Hi @cjlt 

By designed, they aren’t meant to switch on by themselves. This could plausibly be explained by one of the four scenarios.

1. A schedule has inadvertently been created, triggering the purifier at various times of the day. 
2. Another person has the purifier connected to their App, that could be controlling it.
3. Another person is using a remote control on the same IR frequency as the purifier. This is admittedly less common in my view and would only be possible if you had two of the same models. 
4. An onboard software fault has occurred, resulting in this erratic behaviour. This is again admittedly less common in my view, but possible. 

For the first scenario, check the Scheduling your air treatment machine | Dyson Community post and check on the schedule section of the app. If there is one, delete it or suspend it from use. 

For the other scenarios, resetting the the machine to its factory state should rectify any one of these. It will remove anyone else from the App, reset the software and revert the frequency back to a singular remote. View their Reverting your air treatment machine back to it's factory settings | Dyson Community post for instructions. 

Pete.