1 you assume he stole something, or you think he did
2 you did nothing at the time
2A and he now is not in posession of the "stolen" goods
3 you ask your mommy what you need to be doing (how old are you?)
4 your mommy gives you a mixed bag of advice, some of which is wrong
5 you are wrestling with what to do.
pretty simple. as tie suggested, you go in a bit early and speak one on one with your sup, and tell him what you know. that he was working in the trailer, and threw in the empty box. dont say more than you know for a fact. Dont say he stole something, just tell them that the situation made you very uncomfortable, that something wrong could have taken place.
as has been mentioned, if he indeed stole something, you are just as guilty as he if you dont say something. and if ups comes looking back, you will be under the cloud of suspicion. best not to even go there.
in my career i have either busted or helped bust several overlabling rings. not because i wanted to, but because it was part of my job.
do the right thing. in the end, it will work out.
d