Surely a boolean that does not exist in a form can only be a variable created in a microflow? If so, you can use the same microflow to trigger any further actions you wish by doing a microflow call at the appropriate point??
If the boolean is a real attribute (of this or any other form) then you can trigger microflows. The On Change trigger will only work if the attribute is displayed on your form (it is a user ui action). However, you can use other events like the Before or After Commit actions. If the attribute is not part of your current entity, you can commit that without committing the current entity, and so trigger the microflow when you wish.
To do this you would need a separate entity (B) that contains your boolean attribute and a link association to your main entity (A). Your microflow should set and commit the boolean, triggering a microflow on that entity (B). That microflow can use the association between the two entities to retrieve the main entity (A) so you can perform your changes.
Your question is a bit vague, what do you mean with 'not on a form'?
Did you take a look at before commit events?