I will take a stab at this. For more discussion, see the earlier threads. It's been bandied about here many times, as many have the same thoughts you do.
I think, traditionally, men and women just communicate differently from each other. Many men -not all- aren't "chatty", especially on the phone. They communicate what they need to (in this case, being available for the meeting) and that's that. I know that's how many men I know would be. "I wasn't planning to be there, but if you need me to I can". Period. "No, if you're busy (since he didn't say "sure", it's interpreted as otherwise engaged or something) I can do it." That's it. The entire conversation. The whole enchilada.
Now, layer onto that concept a couple of other factors. One being that the LDS church is incredibly "proscribed" in the expected behavior and monitoring of its members. This is a church that expects it's members to open up their personal books to monitor tithing habits. (Not sure if this still happens, but it certainly did in the past.) There are proscriptions about drinking coffee, alcohol, using tobacco, etc.
Then, on top of all of the formal proscriptions, there is, for lack of a better word, "peer pressure". The pressure to go on a mission, to be married shortly thereafter, to be active at certain levels in the local church and to be seen within a pretty narrow window of lifestyle: Modest dress, always "nice", polite, etc. What I'm getting at is that so many members I know wouldn't care for another member to see them even socializing with people who are outside of these norms. My experience. Given this, I think there would often be situations where one doesn't "inquire" much into specifics, especially when those aren't offered. Especially with young, single people, whose whereabouts and daily contacts, etc. aren't 100% centered around family.
Now, layer onto that the fact that both men were in..... Vegas. Vegas. A place most known for gambling, prostitution and in general everything that a Mormon must stay away from.
Certainly, the lack of inquiry as to the reason for being in Vegas is a piece of information to consider as part of the whole picture, but there just doesn't seem to be anything else in the picture that points to anything nefarious or other than what it was on the surface. Just my thoughts.