this is no direct help, but a hint how you might find the string with very little effort :
i once had a similar problem, getting some strange message from an unknown mod. since i didn't want to disable one mod after the other (500++ installed, and 150+ of them enabled :-), and also wasn't sure whether and how the problem could be reproduced with a different subset of mods, i looked at the source of ALL mods. this is easier than you might think and only requires one minute of your work and quite some more time for the computer to do its work :
first: make a new directory on a disk with a lot of space (so that you easily can delete it later without messing up your factorio installation and/or its mods folder), and unpack all your mods to that new directory. in windows, simply mark all mods, select "extract/unzip all to xxx", select xxx, and wait a while ... :-)
second: use an editor like notepad++. that editor has a search feature to search for a string in all files in some directory and all its subdirectories. start that search, and wait a while again ... :-)
third: look at a list of all the files which include that search string ... done :-)