What is the host's reaction if, during a meet-n-greet, the dog owner is seen wearing a gun while in our house, and without having saying anything about it prior to entering our property?

I had assumed that carrying a gun (concealed or otherwise) must be legal in Colorado, but it doesn't sound like it is from this:

Title 18: Colorado allows a person to carry a firearm in a vehicle if its use is for lawful protection of such person or another's person or property. [C.R.S. 18-12-105(2)] Colorado law also allows a person to possess a handgun in a dwelling, place of business, or automobile. However, when you carry the weapon into your home, business, hotel room, etc. it must be in plain view. Local jurisdictions may not enact laws that restrict a person's ability to travel with a weapon. [C.R.S. 18-12-105.6] The Act permits the nationwide carrying of concealed handguns by qualified current and retired law enforcement officers and amends the Gun Control Act of 1968 (Pub. L. 90-618, 82 Stat. 1213) to exempt qualified current and retired law enforcement officers from state and local laws prohibiting the carry of concealed firearms.

https://www.colorado.gov/pacific/csp/...

However, questioning or asking a person who is armed doesn't exactly sound prudent, given all the incidents of violence reported on a daily basis.

The best you can do is follow Deb's advice about having the M&G in a public place, although that's no guarantee of safety either these days.

Wow, I haven't experienced this situation and I can understand how uncomfortable it was.

The only suggestion I can offer is to Always start a meet & greet outside (maybe along where you'd walk the dog), make introductions, walk, and talk. Try to assess anything that might make you uncomfortable (like that or other things) before inviting them inside your home or back yard. You can always end a meet&greet outside, where everyone is visible in a public space.

Maybe he was a police officer.

This is a bit late of a response, but I had to chime in as I'm also a Colorado resident. It can be scary to see a handgun when you're not expecting it, but if the dog owner took the right steps, it should make you feel more relived, if anything!

The Title 18 you quoted is correct for all normal citizens, but I am assuming the dog owner has his concealed carry permit. It is illegal in most states (including Colorado) to conceal a gun under clothing if you do not have this permit. Not anyone can get their concealed carry permit. You must go through training and extensive background checks to qualify.

Sorry for the long answer, but I hope this helps to put you at ease!