Someone on LVA (I think it was Rockin Rick) posted that he thought it was a bit more than unusual that someone would find two envelopes at McCarren, wait until he gets home to CO, and then opens them to find $10K in cash. If you weren't going to open the envelopes when you first found them, what would possess you to take them with you? Sounds a bit strange to me and I think he makes a good point.
I know that I would have definitely called Metro and/or airport security and told them in general terms that I found a rather large cache of money. I wouldn't give them particulars such as the amount, what they were in, where in the airport it was found, etc. In that way, presumably the rightful owner would be the only one who would have been able to give an accurate enough description of the amount he lost, the envelopes it was in (two from Caesars Palace) and where approximately he would have lost it (say for example he was flying on Delta out of the D gates). If I was still skeptical, I'd ask for proof that he had stayed at Caesars (copy of hotel folio) and flew out on Delta (copy of eticket) just to make sure that it was getting back to the rightful owner.
If in the end no one stepped forward, I would keep it with a view towards donating some of it charitably (no, not in a vp machine! LOL). I don't care if you believe it's karma or coincidence but good things have happened to me when I've "done the right thing". Not to be preachy by any means but ask yourself what would you prefer if you were the person who lost the money.
Dan