In this context, in passing is referring to something that was passed by quickly, no time was taken in examining or noticing it. For instance, if you are in the grocery store and you go straight to the back because all you need is milk, you would still see fruit and maybe coffee, canned goods, and so on; but you only see them in passing , meaning you do not stop or even slow to take a closer look at them because they are not what you are looking for. It can refer to people too, if you do not stop and take the time to meet or greet them.
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