lol! This reminds me when I have dropped off my Xbox360 in a plain white box, and the ups delivery personnel said, "Let me guess, your sending this to Microsoft?"
The delivery person, may not know exactly what you have, but surely the package states it's origin which is China.
My uncles a retired Smsgt now, USPS mailman. He told me a little about how things often work when I (once upon a time ago), wanted to start juicing before local bodybuilding competitions (I never did). There are many illegal substances that get imported from China to homes on frequent basis such as roids, hormone pills/injectables, rare illegal ivory carvings, and so forth. Surely, those of us who order many items from China frequently may raise some degree of concern. Eventually, some of our packages will be opened up, (aside from random routine inspections that dogs cannot pick up on,) not just for the reasons of it's origin, but the frequent areas/homes the packages are destined to.
Today's technology applied in today's postal services, allows postal services to corroborate with officials very easily. That nifty little barcode on the stamp/mailing label allows for not only tracking of the item but where it's from and to where it going to. It's tallied in the data bank and if enough packages from questionable areas out of this country come up under one address, well, obviously, it stands out and is reported.
I don't think your packaged had been opened though, the mailer may just assume that it's something either unsafe or questionable since it's from China.