Your comment reminds me that I never told you folks here that after a couple of weeks, I figured out what caused Alexa to talk in her sleep. Would you believe "user error"?!
I was sitting nearby Alexa about 10 days ago when Alexa did it again, and this time I understood enough of what it said that first time and could tell this time it said the name of my PC, and it was because I had just turned on the PC Bluetooth.
Apparently, I had left the PC Bluetooth connected to Alexa accidently a couple of weeks before. The funny thing is, I had turned it on BT the day before to help a forum member who was having trouble with Alexa, because it kept wanting to connect to his PC's Bluetooth and was talking. I helped him get it fixed (turn off PC Bluetooth), but I had also caused the same problem to myself.
Reason my BT was not turned off for a few days is that my PC is mostly never turned off; I keep it in sleep mode overnight or away from house a few hours and reboot occasionally. So, when I awoke my PC from sleep, it also awoke Alexa, too.
OK, ya'll got that? It was caused by my accidentally leaving the PC's BT on, but not using it.
This likely was your problem, too. We all have to be more aware of BT signals triggering Alexa when we don't want it too. So, go around and turn off or un-pair all BT devices on phones and tablets, etc. I recently discovered a way to turn them all off at once in the Echo's settings. Go into the settings of (yourname) Echo, go to the Bluetooth section in there and find the button to un-pair all BT devices at once. That will kill the culprit if you could not find it. Next time you go to use a BT device you will need to pair each one you use.
Whew, glad that mystery was finally solved.
Mostly, just remember to turn off BT on a device after you finish using it with Alexa.