Backup sensor beeps pitter out

OK I just had a really weird problem and searched the forum but couldn't find anything on it. I was working on the truck out in the driveway earlier and then backed it to the garage. I noticed the yellow warning blocks show up on screen and heard a weird sound. I put the truck in park and then reverse again. The beeps started like normal but then got weaker and pittered out after about the 7th beep. The screen still showed the yellow warning marker. I turned the truck off and then back on and went to reverse again and the same thing happened.

The only thing I was messing with under the hood was one of the grounding bolts on the passenger side since that's where I have my OEM grille marker light add-on grounded and I just wanted to tidy it up. Does anyone know where the beeping for the rear sensors is grounded? If it's up front, then it was probably me but has anyone else had this issue?

 

For anyone that is wondering I tested all the sensors. I apologize for any confusion. Apparently this is the normal function if the truck remains stationary so as not to continue beeping incessantly. I guess I never noticed before. Maybe there was some kind of update last time I was in for service.