Thanks for the quick response Vitor. Update testing some of your ideas...
What is actually happening is that if Chrome is not already running, triggering a web search from Alfred is launching two instances of Chrome. It launches Chrome with the correctly-triggered search first, and then immediately launches a second instance of Chrome straight over the top, with the default home page. This happens so fast that I hadn't spotted it at first - and it is actually a new instance of Chrome, i.e. a new window, not a second tab in the same window.
This doesn't happen if Chrome is already running, it just opens a new tab as it should with the correct search. Note that I (and my wife) do have our browsers set to trigger new links to open in new tabs not new windows, so theoretically Chrome should never launch a second window.
I set my default browser to the latest version of Chrome on another Macbook and am finding the same issue. I disabled ALL extensions so I don't think it's that.
And now that I check - yes, this is happening with other Alfred searches as well. Pretty much anything that launches a URL into Chrome - whether it's "Maps" as above, or a Searchio! search (any search, not just a Searchio! m search), or even just a random web search like "Apple Insider" (tested using both Google and DuckDuckGo).
Obviously a simple ⌘+W will remove the offending second window, but still a minor irritation since it's every search where Chrome is not already running.
If no one else is experiencing this issue then I guess if it's not Chrome extensions it may be something else I have installed during setup...I certainly think it's possible that there is something that I install on macs as standard setup, that I would also have added to my wife's configuration. A screenie of the installed apps is here.
By the way - thanks for the tip on Searchio! - that is AMAZING and I am going to use it (with Safari!) on my other macs.
Thanks again!