I was in KC MO from 7-25 through 7-31 and stayed in the Marriott Courtyard Suites. One morning I saw a youngish man OCing in the breakfast line. I was with a group and could not break away to introduce myself and find out if he was LE or not. Days later, I spotted a notice on the front doors of the hotel that "concealed" firearms were banned per MO law. I just wondered if the fella I saw had found a way to carry while staying within the law. The sign did not say that firearms were banned, just the concealed ones. Pretty crafty if that is what he was doing. Don't tell the antis about the "loophole" if my suspicion is right.
It's not really a matter of "staying within the law". Missouri is an "open carry state" and the practice is legal anywhere (with a few exceptions) in the state unless it is restricted or prohibited by a local ordinance in the specific jurisdiction you happen to be in at the moment. A sign posted on the door of a privately owned establishment, regardless of what it says, does not write/enact new law, so it therefore has no force of law. And it's not a "loophole", it's just the way the statutory process works.
So as to the rest of the points in your question - Carrying of concealed firearms into a hotel is not "banned" under any state law, provided you have a valid concealed carry permit. If you don't have a valid concealed carry permit, then the carrying of concealed firearms is prohibited EVERYWHERE, save for the passenger compartment of a motor vehicle, or on property over which you own, lease, or control, or when you are engaged in the lawful pursuit of game (hunting) and also possess an exposed firearm or projectile weapon.
As to the sign, as I mentioned above, a "no firearms allowed" sign has no force of law in Missouri, whether it specifically mentions concealed firearms, or just firearms in general. It would be no different than if the hotel had a "no green shirts allowed" sign on the door. To walk past with a green shirt on would not be a violation of any law, but rather, it would be a disregard for the owners wishes. The owner is well within his/her rights to ask you to leave for any reason, whether they have signs posted or not, so the sign doesn't provide them with any "rights" they don't already have.
As a matter of opinion, I believe common sense would dictate that if a business has a "no concealed weapons allowed" sign posted on the door, then it is their wish that you NOT bring in a weapon - period. Pushing that envelope with a technicality, ie: openly carrying the gun because the sign only mentions "concealed firearms", only serves to circumvent the property owner's wishes. The next step the property owner is likely to take is a sign that says "NO FIREARMS ALLOWED", so as to cover ANY carry mode.