I had a similar worry (I tend to worry a lot)--My coop has a side door for "human access" and it opens the entire side of their coop.
So far, no one has managed to get past me
I keep a sharp eye on them (I only have 5) and if I have to have the door open for any length of time, I find that a tossed handful of mealworms are a good distraction for them.
They're much more interested in hunting for snacks than making a break for it!
However, I do most of my business with the coop (filling feeder cleaning, adding bedding, DE, etc.) when they are out in the run. They may poke their heads in to see what I'm doing in there, but soon lose interest once they see I'm not doing anything that involves treats.
My yard is completely enclosed with a 6 ft high fence, so I'm not TOO worried if they get out (and eventually after they have begun laying, I'd like to "free range" them in my yard.) But I don't think my girls would stray more than a few feet from the coop anyway, and I could easily entice them back with mealworms (I buy them in the big plastic tubs! LOL)
If I had designed my coop, I would have made a "dutch door", I think it's called--allowing me to just open the top half to gain access (my coop is small!) I had thought about tacking a blanket up so that when I open the door, the blanket covers the lower half of the doorway, but so far that doesn't seem necessary.