Are you out in a rural area? My neighbor's birds regularly visit my yard because we have better food and forage lol. We are in a residential area bordering on rural, so chickens are pretty common in the area in general. People very rarely dump hens, if it's a hen your birds could have called her in when calling to each other. If the bird was carrying anything your birds have already been exposed, so I honestly wouldn't worry too much, just keep an eye on them for any signs of infection, pest and illness alike. I have an Amber link hen, and I can say she gets out more than any other bird, I had to clip both wings to keep her in and I have a tenth of an acre just for them. Personally, I would let the bird be. If it wants to join your flock it will, given a chance. It could just be visiting to look for snacks, I definitely wouldn't eat it just because it showed up. It very well may be going home to roost at night if your neighbors are not that far off.
