Or it was just an attempted theft and they didn't think it was something that they could bother trying to sell because they didn't know who would buy it.Damn, sure does look like someone sliced it open with a knife. If that was done by one of there machines that would be one dangerous damn machine.
Seriously though I would not put much worry on it. The TON's of mail they handle every day? Unless its someone actually delivering to you it would be really hard for them to look for one persons stuff. That is weird how that damn thing is sliced clean across like that. Wonder if maybe it got wedged in something and an employee did that to free it? Odd that if they were going to steal they would do that and then not steal the stuff inside.