Package still in Las Vegas, going to St. Louis. Been in Vegas for 4 days, was sent priority 2 day mail. I AM NOT COUNTING SUNDAY! Lost mail?

Give it time, remember it is the Holiday Season and mail will take 2-3 days extra than normal. You will more than likely have the package delivered before you receive notice of it. The post office only updates their system ONCE a day, generally around midnight. Just relax if you don't find anything for a week, then file a lost package search with the post office.

Package is not in Las Vegas. No update means the parcel is moving and has not yet been sorted in another post office. Probably was delivered today.

Just because the online update says it's in Las Vegas, that doesn't mean it's still there. It means that was the last place a postal employee had his or her hands on it to scan it before it got on a truck to go to St Louis.

It might be on the road in a cargo truck or even in the mailman's vehicle as he drives to the neighborhood it's going to.