Not being a lawyer, nor playing one on television, I have to wing it.
The general rule, which is easier to remember than the various specific situations, is that anything you do in public (eating in a restaurant, driving, talking on a cell phone) or leave laying around in public view (like a drug pipe sitting on the front seat of a car) has no reasonable (important qualifier) expectation of privacy.
The envelope question is a bit tricky. If I arrest someone who has clearly stolen a sealed mail envelope (say, a bunch of Social Security checks), I wouldn't be in any hurry to open it. If a US Postal Service Special Agent needs to get into the envelopes for investigation or prosecution of the thief, that's their job, not mine.
If a person is arrested carrying a sealed envelope that I have no reason to think is stolen, I will always open it, which is well within my authority (but different LEOs have different authority). Fourth amendment still applies, but is significantly reduced after arrest. Virtually anything in a arrestee's immediate control is subject to a search to include ripping, breaking, tearing, or cutting to complete the search.
In the house search example, it depends on many factors. If someone is living in an area with the owner's consent (roomate, hotel guest, girlfriend/boyfriend), that person may give consent to search any area the person is allowed to be. If a specific area is locked with no key, that area is likely considered private by the owner, and the Kato Kaelin houseguest would not have authority to give consent to a search of that particular area. If platonic roomates (doesn't include MTV's Real World
)share a house or apartment, they can give consent to search their own bedrooms and common areas, but NOT their roomates' bedrooms.
Conversely, a hotel owner cannot ordinarily give consent to search the room of a guest, nor a landlord give consent to search a renter's house/apartment.
Can a neighbor with a key who isn't living at a house give consent (absent any other condition that would justify an entry)? If that is the case, I'm a little surprised, but not shocked. Giving your car keys to someone certainly opens that car up to a consent search--something I deal with more often than the house scenario.
There is also hot pursuit, emergent circumstances, signed search warrants, Terry frisk, search incidental to arrest and continuing crime that legally justify a house/apartment/hotel room search when consent is not forthcoming.
By leaving a fingerprint or DNA somewhere public, you have no reasonable expectation of that print or DNA being private. Anybody could pick it up--and conceivably put it in THEIR database.