The Hotel, Mandalay Bay, Tropicana, Planet Hollywood, Montecarlo, New York New York, Paris..these are the one I know that do not require elevator keys and that do not have security at night

I don't believe the Hard Rock has any security at the elevators either.  I know it is not the end of the world to meet that special someone in the lobby, but I really love the nervous energy you get in your room waiting for that "knock knock", then peering through the peep hole to see that you have hit the jackpot (because you did your homework on TER).  That is the best.  I much prefer hotels that don't have security.

On a similar note ~ does any know what is the situation at the Palms?  Do they have a security guards in all there of the towers - or just some?  I will be staying there soon and would like to pick a tower without a security guard!

Here's the "old" listing. Got it off the NSA website.   I was hoping to edit it with updated info.  Haven't done so.  If anyone has updated info, PM me.  But I'm only accepting info from reliable sources.....  Problem is, though, security measures are subject to change frequently.  

Hotel Security Notes
The M Resort- No security.
Four Seasons- Security at the elevators after 6pm, sometimes they are always there, but very nice.
The Hotel- No Security
Mandalay Bay- No Security
The Luxor-. No Security but elevator requires a key to access a floor.  East and West wing towers do as well now
The Excalibur-. Security and sometimes very rude
The Tropicana- No Security
Hooters- No Security
The MGM- No Security at the main elevators.
The Signature- Have to pass a guard at a gate so need your (real) name called down
The Mansions at MGM- Key Entry only
New York New York- No Security but some towers require a key to access a floor.
Monte Carlo- No Security
The Bellagio- Security but very nice
Planet Hollywood- No Security.
The Paris- No Security
Bally's- No Security
The Westin- No Security
The Platinum- No Security
Hilton- NO Security.
Ceasers Palace- Security at some towers but usually nice.
Bill's Gambling Hall- Not sure about now but when it was Barbary Coast, they would try to take pictures and have been known to chase you to get one, not kidding.
Imperial Palace- No Security
Harrah's- Security and very rude and ask for ID's however there is a secret back way up that isn't that hard to find and by passes security.  Also if you see the lady before 8pm, she can access the elevators near the parking garage entrance.  If she goes to the 4th floor, she can get to any tower  
The Flamingo- No Security
The Mirage- Security after 9pm or 10 but always sweet.
T.I.- Sometimes they have Security and other times not.
The Wynn- Security after 9pm but usually no problem.
Encore- Security, need to show them room key.
The Palms- Security all the time.  Sometimes they are accommodating and nice, other times rude and questionable.
Palms Place- Need a key card.
The Rio- No Security for the most part but sometimes they do, usually nice but can be judgmental so best to meet downstairs.
The Orleans- Security but nice.
The Gold Coast- No Security to my knowledge.
Venetian- Security after 9pm on the 2nd story but requires key to enter and always security on main level.
The Palazzo- Security
Circus Circus- No Security
Riviera- Security at some towers but usually nice.
Sun Coast- No Security
South Point- Security but never a problem
GVR- No Security
Golden Nugget- Security after 9pm but sweet
Wild Wild West- No Security except patrolling the parking lots which is how you enter the room.
Palace Station- No security to my knowledge.
Trump Tower- Need a key card to access elevators.
Red Rock- No security but key access only.
Aliante Station- Not to sure, I have yet to go there.
JW Marriot in Summerlin- No security.
New HRH tower- Security
Aria- Security after 5pm, best to just meet downstairs IMO.
Mandarin Oriental- Security after 5pm, best to just meet downstairs.
Vdara- Security after 5pm, best to just meet downstairs.
New tower at Plant Hollywood- Not sure, if anyone knows, please let me know.
Hotel 32- Need key to access the elevators that go to the floors.
Tuscany Suites Las Vegas- Need a keycard to access the halls.
Marriot Grand Chateau- Must have keycard after 7pm.
Cosmopolitan No security at the main elevators.

btw, I was only joking about the NSA...well...sorta!  LO

Key cards are not needed to operate the elevators.  Security for elevators in the evening, starting around 8 PM, but often times they're not there... nice and friendly.