Bluez- while anything is possible in dealing with the govt, have not encountered that.
In the case of NFA firearms (class IIIs) the law requires (under some circumstances) that a contraband item be surrendered to the BATFE.
In the case of kids are cleaning up the house after the funeral, and find that Dad did really bring home a Schmeiser machine pistol, and put it in the closet- and in a reasonable time after realizing what you had it, you contacted them- no, they have actually been pretty decent about it. Happens more often than you would think.
DISCLAIMER- not a lawyer- gun collector, 03 FFL holder, broken down old cop- but to commit a crime, there are 2 elements- mens rea, and actus rea. Mens rea- you had the mental INTENT to do the act, and you actually carried out the act.
You go to a mini-storage auction. For $10, you buy the contents of unit #34. 5 large carboard boxes. One is flled with heroin, one with Uzis, one with counterfeit cash, one with Twinkies, and one with body parts of former competitors. While there are potentially more Federal and State felonies here than a law class could list, unless you take some further actions (trying to hide the contraband, or sell it to the Heck's Angels MC) You have no displayed INTENT to illegally possess any of this.
You may recall that I advised the OP to contact the Firearms Branch soonest- as in first working day. Best way to demonstrate lack of intent is prompt notification. If you are REALLY worried, you can always find a REAL attorney (which again, I am not) and have your attorney notify the agency.