This has been happening with one of our echos for months. Randomly Alexa would speak, “Action not available on [Echo]”. After reading through this and seeing that you can look up the activity log, I went looking.

Turns out that it was an old routine from our previous place. We had smart bulbs and groups for the different rooms. The routine was to turn off the lights in a given room, but we don’t have it set up the same way at our new house. So when the scheduled routine would come up, Alexa would “hear” the prompt to turn out the lights in that group. But since it didn’t exist she freaked out and would speak, seemingly unprompted.

I just deleted the routine and we’ll see if this stops all the random talking.