I believe just about all Text Viewers/Editors have option to search for specific string, but from multiple files then I don't think many programs can do it unless the program has built-in supported MACRO/Script (whatever they call it) then you should be able to create a Macro to switch to NEXT FILE to continue the search.
I came from TEXT DOS so Windows GUI is pretty new to me to know much about Win apps, but if you know DOS program like QEDIT then with a simple macro (I haven't done it since switched to Windows world) then it can do much more than searching for string from serveral files.