Can anyone tell me under what circumstances does a call like this

customer.Orders.Remove(someOrder);

persist the changes to the DB immediately? I am making a change to the object and I am 99% sure I am not calling session.SaveOrUpdate.

Can someon verify if this is possible to do? I've been spinning my wheels for hours and I have checked and re-checked to make sure I am not saving.

If it helps, can someone explain this? What does "value type" and "entity" mean in this context?

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If an object that was removed from a collection is an instance of a value type (eg, a composite element), that object will cease to be persistent and its state will be completely removed from the database. Likewise, adding a value type instance to the collection will cause its state to be immediately persistent.
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On the other hand, if an entity is removed from a collection (a one-to-many or many-to-many association), it will not be deleted, by default. This behavior is completely consistent - a change to the internal state of another entity should not cause the associated entity to vanish! Likewise, adding an entity to a collection does not cause that entity to become persistent, by default.

The logs would help in this case. All changes to collections are only persisted during Flush, but Flush may be done implicitly during some operation (most often before executing a query).