Greetings!
I am a bit embarrassed to admit that I am creating this post, partially for me to find it again in the future, if I end up forgetting (again) that this is a ‘solved’ problem for me! 
But maybe other users might have a similar request/need.
In the course of my using DT3 as a research tool, I might have several (ok, many) windows open, all concerning a particular topic.
As I am writing over in Scrivener (wherever), I am jumping between multiple DT3 windows, and reading/annotating/writing away, as I go.
It frequently happens that I might read something – but only realise a few minutes/hours later that the particular point was more relevant than first anticipated, and I want to find it again, to reference back to/insert into my writing.
Whilst DT3’s search engine is wonderful, I know the part I am looking for is somewhere within the multiple windows I currently have open.
AFAIK, there is no way to search through only those open windows.
What works for me:
Jim helped me with a script to add a tag(s) to all of my already-opened windows.
I still use this frequently, when I want to be able to continue working with all my already-open files over on DTTG. I simply use the script to add a {*Temp} tag to all of them, and can then “access” those same files by using the Tag-search over in DTTG.
Not perfect, but it works.
Using the above, were one to run a search for that term that was recently read, but then simply add a/that specific tag to all the open-windows – and then use that same tag as a further search-filter, you effectively end up with the desired result: A way to search for something specific, but only in those already-opened windows. [Btw, this script allows you to quickly remove a tag from all selected files – which works a charm when I am done].
Figured I would pop this up (for me) and maybe someone else who needs to do the same.
It’s a bit of a kludge, but works! And please, if I am missing something blindingly obvious that might do the same, and works for you – please share!