Thanks for the info. The device must have (mis)interpreted something I said as a request to set an alarm since I never used the word "alarm". At night I usually say "Sleep timer, two hours" or something like that. The other day I probably said something like "Play XXXX radio station for 15 minutes" and Alexa took "15 minutes" to be an alarm request.
I just now did a little test:
"Alexa, one minute"
-- Hmm, I'm not sure.
"Alexa, for one minute"
-- One minute, starting now.
And then there was the alarm sound after 60 seconds. So it's the phrase "for {time}" that Alexa is interpreting as the request to set an alarm.