This seems to be similar to #1230 (closed), but in that case I can see the triangle menu buttons where one would close these. They aren't present on either of the toolboxes in my case. I just installed gimp on a new computer, switched to single window mode, then chose to unhide docks, and noticed two toolboxes. Here is what things look like after a close/re-open (just to be sure it wasn't due to my current session):
I don't know gimp well enough to know which possible windows > dockable dialogs > foo or other option I'm looking for. This is on gimp 2.10.28, Windows 10 64bit. Feel free to ask for other details and I'll happily provide them, and thanks for any suggestions. I'm perplexed!