I'm a USPS carrier and while not all int'l shipments have to be signed for, certain national postal services do require this for all transferred shipments, and BPost is one of those (but e.g. Canada and British posts don't require it). You can't scan these packages delivered without a signature so if they're being left and you're not signing, it probably means your carrier is signing it as an "authorized agent."
Also these packages are considered express with a guaranteed delivery by 3 pm the day they're scanned out for delivery (which is pretty funny since there's no actual guaranteed delivery date...) and bc of that and being big and heavy, it's often a supervisor who will deliver them while running several express packages, and not your actual carrier. That's probably more often the case in urban or suburban areas. The station manager at the post office that covers my house was until recently the station manager at the station where I work, and has recently taken to just signing for me when he sees them which saves me a trip.