Today I received an envelope that was opened and empty. I went to the PO and showed it to the Postmaster. She said you never should have received it like this to which my reply was Duh. She said it should have been stamped no contents inside and then proceeded to stamp it and handed it back to me. I told her thanks, I feel much better now.
If anyone sends envelopes like this what I do is place a piece of packing tape over the flap to avoid it from being lifted open.
Barry