Then again, I just remembered that people are already walking around with listening devices in their pockets that are often always monitoring what is being said ("OK, Google!"), and I haven't heard anybody suggest we need notification. I guess when we are constantly recorded, there isn't much point in notification. Privacy is the exception, not the rule.
Also, almost all cellphones (and perhaps tablets), even some older ones, have a baseband processor, which is listening to things you say until you remove the battery. In newer models, they can also continue on/working some hours after the battery has been removed.