Reliable sensing method for device inside a parcel to detect when/if the parcel is opened?
Wrap a small 40 gague wire all around the package with a microprocessor on the inside and a way for the microprocessor to detect continuity of the wire. The microprocessor could log the time that the continuity was broken and you would know that the package has been opened. With a small wire one could also hide it between layers of packaging tape so no one would see it if they were opening the package. 

One could also place the microprocessor on the inside of the package with contacts to the outside to make for easy installation. You cal also get microprocessors with encrypted firmware so it would be difficult to modify the time stamp.