The scenario you present is very possible and, in fact, happens to varying degrees on most rivers that have hatchery summer runs.

Straying is nothing new and in the case of summer runs, the reasons for straying range from lost olfactory(smell) of their natal river to curiosity because they are so far from being ripe. Whatever the reason-it does happen.

Whether to target a river for straying fish is a gamble. Yes, it may get the odd fish but you will likely put in many fishless miles searching for one. So, you have to ask yourself if that time invested is well spent with better producing rivers nearby? Or is it worth it for the solitude you will likely have even though the odds for connecting with a stray are minimal.

I have had random encounters with strays on 5 diferent rivers over the years that don't get planted for summer-runs. In none of those cases do I feel compelled to target them for strays as the feeling I get was they were flukes; not to mention I fish them regularly for other species and have seldom encountered additional strays.

Just food for thought!