Reddit reviews ASR Federal Ultraviolet Reactive Invisible Theft Detection Powder and Portable UV Flashlight

UV tracking dust

It's tricky, because it doesn't leave a trail per se, but you can see what they've touched.

If they are leaving the entire printed page, see:

https://www.eff.org/pages/list-printers-which-do-or-do-not-display-tracking-dots

You can also try this and dust a teddy bear memento or dust the holder for some flowers then pay suspects a visit with the flashlight

Seems likely to be something like this.

Here you go:

Designed for theft detection and the identification of stolen or altered items


Powder, once touched, remains on fingers and hands, invisible except under UV light


Ultraviolet powder and paste can be applied to locks, fire alarm boxes, off-limits door knobs, computer keyboards, etc.


Non-toxic | 1/4oz. container


Powder & Flashlight Combo Pack

Actually, biologists use UV light to find rodents as well - we just use powder. So you can try baiting them in and using a powder that glows in UV to track where they go, and maybe even where they are coming in - though I don't suspect they are using that right now. Probably just happy to be warm and dry.

Edit - It's a little pricier than I recall, but I suspect you can find something cheaper, this was a quick search.

I mean, the concept isn't entirely divorced from reality, so long as you believe the attack CTUs have extremely good foresight and preparation

https://www.amazon.com/Ultraviolet-Theft-Detection-Powder-Flashlight/dp/B0038AM1QU/ref=pd_lpo_194_tr_t_3?_encoding=UTF8&psc=1&refRID=P6076T3AECBTFYZRH6C6

I think it could explained as the spray having a "dripping" or dust component that could be seen with a black light (perhaps why Joe wore that mask). You leave a little behind everywhere you go. Joe already monitored them and knew where they'd be on purge night. He could go to their location and follow the physical trail. You can buy something similar on Amazon, so it reasons that the military could have a stronger, more potent version in the purge universe that leaves a trail.

https://www.amazon.com/Ultraviolet-Theft-Detection-Powder-Flashlight/dp/B0038AM1QU

1. Put some books or whatever in a box and address it to yourself.
   
   
2. Dust the outside of the package with a powder that is only visible under black lights. Here is an example, although they charge you for a flashlight and it's expensive. I'm sure you can just Google and make your own of whatever it is.
   
   
3. Put the package in your mail room right after the mail is freshly delivered.
   
   
4. Once it's gone, use a black light on everyone's doorknobs/see if you can find a trail going to someone's door.
   
   
5. Once you figure out who it is, notify the police and/or USPS.