For all the GF Fans, we lost another one this week. Rest in peace Alex Rocco...

http://www.cnn.com/2015/07/19/entertainment/alex-rocco-godfather-actor-dies-thr-feat/

See the Dead Pool thread in General Discussion.

in the Las Vegas scene, there is a button / bodyguard standing standing by the door after fredo kicks the band and the girls out he's not seen in the hallway when mike and company are walking from the elevator to the room, but in the next scene he is in the room. he has a gold turtle neck shirt and gold and black houndstooth coat on. he's not with mike or fredo, so he has to be one of moe's guys because he leaves when moe leaves. the thing I don't get is, if he is moes' guy, why does mike discuss this personal family business in front of him. surely mike wouldn't / shouldn't be this sloppy, or just directorial license?

Well, you can never rule out directorial license in any detail of the Trilogy that doesn't make sense. Plus, turtlenecks with sports jackets weren't a fashion trend until the late '60's, and that scene was set before 1955.

I'm guessing that the guy was hotel security, summoned by Fredo to make sure no one crashed the party he had planned for Michael with the girls and Angelo, the accordion player. He may not have specifically been Moe's guy--just an employee. You're right: Michael logically shouldn't have discussed business with him around, but maybe he just ignored the guy. Or maybe he figured the guy was paid to keep his mouth shut. Or maybe it was, as you said, directorial license--people were still wearing "dress turtlenecks" when GF was filmed.

There are other scenes when people talk with "strangers" in the room. Is there any code by which security people keep their mouths sht about what they hear .."or else?"

A ha! He was Michael's informer in Moe's camp!