i'm often baffled at the work credentials i see displayed on the train. it's not very often i want complete strangers knowing where i work and my full name. while the likelihood of any negative consequences from displaying such information is remarkably small, it's remarkably smaller when not displayed. but, more importantly, lanyards and belt clips make an otherwise professional looking person look sloppy (though, admittedly, "otherwise professional" isn't always applicable).
whatever the safety and fashion implications of work credentials are, i don't think i'd be alone in thinking they are magnified many-fold when you hold up a big, fully-addressed envelope address-side out.
who knows? maybe she's just delivering this to the post for someone else. maybe she's part of a criminology research project to see just how dangerous flashing your full-name and address together in public is. maybe she just don't care. and maybe she's even trying to be discovered and wants to be stalked. any of these might be true for her. but unless they're true for you, please PUT YOUR CREDENTIALS AWAY and protect your pii (that's a census term, google it).