...one other thought, and this is gonna date me horribly.
As a kid growing-up with just 2-point seat belts in my parent's car (and able to legally sit in the front passenger seat, not really seeing very well over the dash), there were no buzzers if you didn't connect the belt. My Mom used to always ask if I had buckled-up or reach over and give it a tug. Then, at some later time when I started driving and had my new-at-the-time Fords, Toyota and Hondas before what's listed in my sig line below, 3-point seat belts along with the passenger seat "in-use" weight detection method became the norm. The latter for sure would make the buzzer go off if you had too much weight of anything in the passenger seat. Some cars were really touchy to the point some other drivers became so frustrated when they threw their purse, a grocery bag, or something else on the adjoining seat, they just keep the passenger belt clipped in-place (or used an aftermarket device to trick the clip, or had DIY methods to clip the wires or put in their own switch) to avoid the buzzer.
After that, maybe around the time child seats and if they should/shouldn't be in the front seat and face back or forward -- while airbag tech was also coming into being, I distinctly remember in one of my cars there was a not-so-obvious method described in the owner's manual to permanently disable the buzzer related to the weight detection in the passenger seat, so it would only keep the red dash indicator light on if an exception was found. I never did it myself, but being a guy that has always read the Owner's Manual at least once, remember it was at least that mfgrs response how to deal with the myriad of exceptions that existed at the time. On perhaps my next new vehicle not many years after that, there was no way to turn off the buzzer if I even wanted to, which I remember thinking at the time was odd or some sort of takeaway. I always suspected the change was because some new safety legislation had been passed, or maybe improvements to the weight detection methods in the seat had been made so it wouldn't need to be manually overridden any longer.
It appears from @jaytang's observation, Volvo is now just automating the buzzer turn-off after some period of time, that in Volvo's wisdom accomplishes the reminder the buzzer was intended for in the first place, without providing a manual override to defeat the warning system all-together -- but also makes things better for the outlier cases I suggested above and passes whatever legislation there is requiring 3-point belts, the "seat-in-use" weight monitor, and buzzers that have to go off for some minimum period of time when an exception is found.
I didn't feel old until getting into this thread a few hours ago, but now I sure do!
Perhaps better reasoning may be that since the Volvo XC design has gone to all the trouble of having the built-in holes just under the headliner above the rear doors to accommodate the optional steel grille -- at least in the XC60 (to protect the front seat area from animals or packages falling forward -- but I suspect only a small minority of owners here in the US actually ever use, since most other mfgrs provide nothing similar or at least as visually apparent), that could be why. IDK for a fact, but its a pretty good speculative POV off the top of my head at this hour, isn't it?
