Some things that raise suspicion is a package with string or rope securing it. A package with too much postage afixed. A package that is wrapped messily or does not close properly. A package with no return address. A package that is leaking especially with grease or oil.
I found these listed in our office and it was supplied to us by the Postal Inspectors who ask us to keep an eye out for these things. Some of the items listed are signs of a package bomb and that would definitely cause suspicion. Always go into a Post Office and pay the correct postage if you are mailing a package because if you just throw a ton of stamps on it and throw it into a mailbox it MAY be deemed suspicious.
Otis