DETECTOR MADNESS, WHAT'S GOING ON?
I will try to summarize this issue as I best understand it. Here's what happened:
First, when the Mario Galaxy collab went live in Japan on April 24th (it was delayed compared to the rest of the world to match the movie's release schedule), some users experienced a problem when using their Detector. For the duration of the event, using a Detector when the Movie Theater icon appears should guarantee at least one seedling from the Movie Theater category regardless of how many other symbols appear simultaneously. However, some players were not receiving this guaranteed seedling as anticipated.
The solution: Niantic decided to compensate players by distributing a Gold Movie Theater seedling to all players. In addition, they gave Detectors depending on how many were used (aside from the free daily one) during this period of error, which differed per user.
The second problem: Something went terribly wrong with the compensation of these Detectors. Players were reporting that they received 5, 10, 20, 50, 100 and in some cases over 500 Detectors in compensation that was definitely by accident. What exactly went wrong is unclear, but certain players were basically being awarded with a large amount of Detectors that they weren't supposed to get, while other players didn't get any at all.
If it was just one or two extra Detectors by mistake, I think they would have just ignored the issue, but since in some cases over 500 Detectors were given in error, Niantic decided to revoke everyone's mistakenly-gifted Detectors. The result is, all erroneously awarded Detectors have been rescinded from affected accounts. In this case, affected players will see a popup that says "We have deducted the X over-distributed Detectors. For details, please see your newsfeed."
The final problem: No news update was visible in the newsfeed giving more details as described. This news was meant to go out only to affected players who had Detectors revoked. However, due to the news post not being displayed properly, the news titled "Regarding the deduction of excessively granted compensation items" has instead been disseminated to all players, regardless of whether or note they were impacted by the Detector removal. If you see that news but didn't get the initial popup about removed Detectors, don't worry, you can just ignore it.
As the final compensation for the initial failed compensation, all players will receive 5 Detectors tomorrow on May 2nd, Japan time.
*Note that your daily free Detector, any Detectors already in your storage from being won from the monthly event challenges or bought from the shop are not subject to any kind of deduction and should be safe and sound as always. The deduction only applies to the erroneously compensated Detectors. If a player already used Detectors that were incorrectly granted, those also will not count towards any deducted from your inventory. (So if you already used all your mistakenly-gifted Detectors before the compensation, congrats, you won!)
Hope that pretty much summarizes everything but as always, if you have any questions, leave them in the comments :)