I check indexing using scrapebox. I use public proxies because every type of proxy burns out quickly on index checks. I actually write SER public proxies to my desktop, and use the refreshes to keep updating the proxies on each iteration of the indexing run. Granted, this is a goofy way to do it, but I get a live check this way before I start using them again. That way I can pound through this exercise.
I haven't noticed any false negatives, but that is not to say that they don't exist. I will have to check this on my next indexing check. If this is true, then it is just one more thing to fret over.