First thing I did when I first read the story was check my editor. I already had the "Zero Width Characters locator" plugin installed, but that covered less than a handful of specific space character type codes.
Still, the result was good: Looks like IDEA editors like Webstorm show invisible characters with colored background and a warning.
My test was from that first article and also now from this one copy the example code they contained or linked to from the browser into an open file.
Interesting. PhpStorm highlights the variable after `timeout` but does not highlight the variable after `http://example.com/`. Even pressing F2 to go to the next error goes to the first variable (the highlighted one) but not the second.
However, placing the cursor on either does highlight the second.
I'm using the Darcula scheme. Your screenshot obscures the second occurrence, so we cannot see if your light theme has the same issue with the second occurrence not being highlighted as Darcula has.
You are right, I missed the other one, it is not reported. You can see there is something because it takes space, but you have to deliberately go there to see it. There also is no warning from having the "No trailing spaces" setting active, so it is not seen as a space character even if it shows as such.
I'll write an Issue on youtrack, I'm sure they'll fix it. From the well over hundred issues I ever reported about 2/3rd were fixed (rest is obsolete, only a few that are really still open).
Yes, I file bug reports with lots of places, and Jetbrains is one of the best for actually doing something with them. It is one of the few non-FOSS applications that I am willing to integrate into my workflow (hmm, the only one I think).
EDIT (new comment because edit-period is long gone):
It's not too severe an issue, maybe not one at all(?), at least in this concrete example, because after removing the first occurrence of the hidden variable it now becomes a "not defined" real error and not just a warning in the second location.
Some time ago I managed to add a non width character in my PHP code. Because it had no width PhpStorm did not highlight it and I had absolutely no clue why there was an error in my code.
So it only highlights when it has width.
Edit: just added some non-space characters and at least in Rider they are now displayed as a warning. So I think this is fixed now.
While not as fancy, font choice may save you here too. I use vim and while the editor doesn't treat this character as special, my font (Iosevka term) doesn't include this character, and so it's rendered as the generic "missing unicode" glyph with the code inside it.
Still, the result was good: Looks like IDEA editors like Webstorm show invisible characters with colored background and a warning.
My test was from that first article and also now from this one copy the example code they contained or linked to from the browser into an open file.
Screenshot: https://i.imgur.com/ColuRNB.png