I would expect a tracking check from a TOR exit node to be a "clue". Might even be grounds for reasonable suspicion (would need a judge/case law to know for sure). Which is enough to isolate a package for a drug sniff. Though for international shipments I don't think the postal service/border folks need to meet a reasonable suspicion standard for a sniff.
I guess the take away is... never check your parcel's tracking details.
