Here’s a balanced explanation given by Mary Kole, a former kidlit agent, and kidlit editor extraordinaire, about when and only when illustrator notes are to be used:
http://kidlit.com/2010/11/17/should-you-include-illustration-notes-in-your-picture-book/“The point of an illustration note isn’t to jot down every single thing that’s in your imagination. It’s also not to micromanage the potential illustrator. The point of an illustration note is to convey something to the manuscript reader that is not obvious from the text.
Only use illustration notes in your picture book manuscript if there is something integral to the plot that you want the illustrations to convey, but it’s not described or alluded to anywhere in the text. In other words, if I will be blind to something from just reading the text, use an illustration note to describe it, but really do keep them simple, spare, and few in number.”
Kole’s entire blog post is an excellent read and her examples of when to use a note and when not to are great.
That said, in your example, Sashie, I would say that if you leave the spy gear out of the text (which, as you say, would be good to do if the illustration shows the gear) then you would need to put in the illustrator's note. I would think, though, that if you explain in your query that the main character is headed out the window with spy gear in hand and subsequent scenes show him using his spy gear, then the reader of your ms will visualize the spy gear without the note.