You might have a module draining your battery. There’s a long 150+ thread on the Acadia forum about this problem. Some have found water inside these things when removed from inside the rear bumper area.
I’d probably start with a test for parasitic drain, OR - just remove that rear sensor and look at it closely. The answer may be obvious at that point.
UPDATE: LOL! I just saw one of my posts over there ... I *knew* I’d seen this complaint before. Anyway - one poster “Tmcg” pulled his side module and found it full of water, with circuit board inside corroded! Bought a new, pre-programmed, one from RockAuto for $215, and installed it himself. But it *still* needed some kind of “programming” (but he owned a MDI/TECH-2 GM device ... others *may* have to visit a dealership). He also ended up going back to his newly-installed module, re-drying it out, and sealing it up with epoxy and RTV to help prevent a future re-failure.
See post #70 and beyond.
Looks like GM started an SCA (Special Coverage Adjustment) on the Acadia for these failing Side Detection modules (up to 80,000 miles and up to model year 2017), so it’s a widespread issue.
If it were me, I’d try turning the feature OFF in the DIC menus first, then pulling the bad module to clean and reseal it - to see if that worked (or at least kept it quiet)... keeping in mind that (apparently) a mal-functioning module can cause *other* failure symptoms that *could* affect driveability.
(See post # 137 and beyond in the above thread for those “other” potential problems)