Three beeps from blind spot monitor system

I'm a fairly new Tundra owner, and this is my first vehicle with a blind spot monitor. A few times now, it has given me three beeps. For more precisely three sets of three beeps. I'm not sure if it has always done this in reverse or not, but it seems like it. Three beeps and the blind spot monitor warning lights flash. Does anyone know why it does this?

 

Probably cross traffic alert, especially if it normally occurs in reverse

 

rear cross traffic alert, detects pedestrians and cars

 

Thanks so much guys, I wondered if it was something like that. I'm pretty sure you guys are spot on. That's amazing technology! I didn't even know it did that. Couldn't find anything in the manual about it. I was very likely looking in the wrong place.