Odd ring coming from doorbell 2 - what is it!

HI all, we have a Ring Doorbell 2 installed at my mom's house, indoor watching a door/foyer. It has been installed about 3-4 days and so far is working great. She has the app on her phone as do I.Last night an odd thing happened - it started making a rather loud rining sound, a loud tone, about a half second on, a half second off, for about 5 minutes. Neither of us were on the app as far as I know, she told me about it this morning and it triggered the motion sensor because she walked up to the device and looked at it, to see what it was doing - I could hear the audio.Also - no one touched the doorbell button itself. I have uploaded a short audio of the tone that I recorded in QT from my phone/Ring app history.It seemed to happen from nowhere - what caused this?[ring.m4a.zip|attachment](upload://fAV0tQCKBV78swvc14hMHdKm0YG.zip) (268 KB)