Having a background in a vastly different kind of inspection work, we called random inspections "the scarecrow effect." Deterence is cheapest - just like putting doubts in those crow's heads. The "bringback" doubters insist on written documentation, so your informative post might not meet their standards.
"Bringback" is a convenient, but misleading term. "Send homes" would probably be more accurate. The military committed to making mail service as quick as possible and sacrificed security to that end. A statement by the War Department Deputy Chief of Staff in March '45 included a long list of USGI gear found in parcels being mailed home by GIs. The statement went on to say that 300,000 parcels per month were mailed home through the NY POE and another 200,000 through Boston. Guess how many customs inspectors those two POEs had? 4. That's total, not each. Think all of those half a million parcels per month only had cuckoo clocks and beer steins in them? At the time the statement was made, there was no requirement for inspecting parcels prior to mailing (they soon added the requirement - but it didn't work either). With that level of staffing, looking inside 1% would have required a major effort. You don't think the grapevine got the word around that it was a low risk endeavor, do you? Why risk getting arrested getting on or off the boat - and delaying your arrival home, when if you mail a parcel just before embarkation, at least you'd get home before getting nabbed.
Besides the obvious pistols, carbines, and MGs, the statement listed dental instruments, welding equipment, and live hand grenades as being found. If it wasn't nailed down and a guard posted, it got packed up and mailed home. When the army busted the Hesse crown jewels ring, they went to the home of the parents of the main perp, Jack Durant. In an unopened box he had sent from Europe and had addressed to himself they found a 1911 pistol. Customs inspectors manged to miss that one - and no teliing how many thousands more.
Being dubious of a particular undocumented item is a totally separate matter from claiming such occurences were rare.