Me and my gf kept not getting each other's calls, so we were comparing our call logs to try to figure it out when I noticed a few things that perked my ears. I haven't mentioned anything to her but it keeps crossing my mind.

2 weeks earlier she was in vegas for the weekend and I noticed these things:
she had a missed call from a random dude @ 4am (with no return call) a night she went clubbing. She also had a missed call from her sister who was staying in her room with her at 10am that morning.
I asked her who the guy was and she said it was a promoter for some club.

But, she also called me at like 4:30 am and we talked for a while (I heard her gf's come into the room, she got angry when one stepped on her). So, she was probably in her hotel room and she said she was exhausted. She also called me at like 10:45 am the next morning. I never asked her to or contacted her.

Here's the thing that smells of funk: unless her sis was out of the room early why would she call her in the morning?
This raised a red flag for me so I went ahead and called the dude to see if he was really a club promoter. He said no, but he was a long time ago...
So, does this stink of a fling or a failed attempt for some dude to pick up my gf?
Worst case: they met up at like 5am and fucked; she did seem out of it the next day (but also wanted to fuck me when she got back).
Funny enough, the guy gave me his contact info as we taked about throwing parties in vegas, which I'm doing. He's an attractive guy, somewhat of a pua, and looks like someone she'd be into (she's hot and picky with men, as all hot girls think they are).

Any ideas how to get an answer without throwing a wrench in an otherwise happy relationship? If she's sneaky enough to meet with some dude like that and leave no other trace, she would never tell me the truth if I flat out asked her. I hate feeling like damn detective...what would you do? Ask the dude? lol. Forget about it? My gf said she knows I'd can her if she ever cheated (probably because I have options). Is this petty shit?

Don't. Seriously, just hold up now before you make your life a lot more difficult.

First off, you're in a relationship, and I will assume you love this girl. Love needs trust. There is no good way to ask your girlfriend if she is cheating. Here's the easiest way to not worry: it was a MISSED CALL. She didn't answer the guy. How could she have gone to fuck a dude she didn't speak to on the phone. The real worst-case scenario is he hit on her and she gave him the number so he would go away.

Second, 10 a.m. also isn't so early that it is unreasonable to believe that your girlfriend's sister didn't just go out to get breakfast while your gf slept more.

Third, if I was taking a vacation in Vegas, I would seem out of it every morning because you can be damn sure I'd party hard.

You asked, she told you, drop it because if you overanalyze this you will ruin what looks to be a happy relationship.

her sister could have been going out to the car or the ice machine

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what dr. house brings to medicine, i bring to everyday life (an extreme dose of cynicism), don't listen to the curmudgeon!