Start with these ideas:
I own a few Prius, luckily, I have not suffered cat theft and I would like to avoid it. My idea is that I would like to make some sort of anti-theft sticker. I think I want that sticker to point out that it's not worth it; the cat is not a quality/genuine item. Even if it is a quality cat, how can I best make the potential thief think its not worth the effort/risk?
I see other sticker available that say:
But, are these stickers deterrent enough?
I'd like to place a discrete sticker low on the rocker. About where a would-be thief would place the jack, like just above the orange jack shown.
So, here is the question: As brief as possible, what words should be on the sticker?
What would best communicate to the thief that this is not the one you want?
I remember the No Radio stickers of the 90's. For that, the thief would be looking in your windows so the best place for the sticker was the window. For a cat, I'm not sure the window is the best place for a small sticker. Like the samples above, the notice in the window has to be huge/full sheet of paper.
Wow, when was the last time you heard of a car stereo being stolen? In my youth it was a big deal and common occurrence.
In reply to John Welsh :
Back in the 70's getting your CB radio stolen was a big problem.
Blinking red LED.and motion detector under the car ?
This one is hard to improve on:
I'm curious about aftermarket cats that don't have precious metals in them...
Motion sensor alarm seems effective. Wouldn't a cat guard be effective? Can you paint it black so it looks like it's been cut out?
Or a sign similar to this?
I'm not sure a sticker would be all that effective. Definitely easier and a whole lot cheaper than a shield though. The shields that Toyota supplies are stainless units made by a third party. The sheet isn't very thick though, and I've seen aluminum ones marketed too, so I'm not sure how much of a deterrent they are if you're already armed with a reciprocating saw...
The short version is, I don't know if I'd trust a sticker, but maybe it's just as good as anything. Unless your lowering game is super strong.
OHSCrifle said:
Motion sensor alarm seems effective. Wouldn't a cat guard be effective?
As much as I love cats, I think a German Shepherd would be a better guard. :)
The thing I don't like about the sticker above or a gun sticker or even a sticker announcing a cover is that they all sort of say, "I know I have something valuable and I'm challenging you that I can do a better job of keeping it than you can do of taking it."
On the other hand, I like the paper notes above that just sort of say, "you're too late, the good one is gone and all that is left is crap."
Fair point, for that use maybe have a sticker that says "aftermarket catalyst - no scrap value." It turns out that they do have some precious metals as you would expect they would need to function, but far less than OEM models so their scrap value is near-zero. You could make a cat symbol with an "N" on it and a flow direction arrow as these are common signs of an aftermarket cat that thieves would be on the lookout for. If you make a color sticker have the cat in a gray/silver color as well.
In this case I'm assuming it would be a bluff but it could cause a thief to switch to a more promising target rather than taking the time to judge for themselves.
Paint the cat with some bright colored header paint? Either you won't fool them & they'll take it anyway, or they'll just move on & figure something is up.
This bike lock company puts a shell around the lock that contains pressurized gases that when released - only possible with an angle grinder - causes anyone nearby to vomit.
https://www.skunklock.com/
It'd be useful for cats, too.
I don't trust thieves to be smart enough to see a sign, and it seems like much of the theft where I live happens at night anyway. May as well make them puke.
I suspect any window sticker would be ineffective, the average thief wouldn't even notice it. If there was some way to coat the aftermarket cat with bright yellow with a warning note printed directly on it that may work, but it would have to be something that would stand up to the high heat.
How bold of you to assume a cat thief can read.
I hate to say it, but this is a particularly useless thread. A friend had a window broken in his truck to steal stuff, when the door was unlocked.
Your average thief isn't bright enough to understand those squiggly lines.
"you're too late, the good one is gone and all that is left is crap."
I kind of like this one. Don't even clarify what you're referring to, keep 'em guessing. 
GameboyRMH said:
Something like this?
This genius. I had no idea that distinguishing features of aftermarket cats are model numbers that start with N as well as an arrow informing the installer of correct placement.
But, to someone in "the game of cat theft" that N and arrow should be a strong call sign.
A genuine Prius Cat has these small markings:
By the time I have my sawzall ripping, I dont care what a sticker says. Punishment is the same regardless if I steal a factory or aftermarket CAT (none lol) so giddyup!
93gsxturbo said:
By the time I have my sawzall ripping, I dont care what a sticker says. Punishment is the same regardless if I steal a factory or aftermarket CAT (none lol) so giddyup!
sawzalls are for newbs. 
Silent.
P3PPY said:
sawzalls are for newbs. Silent.
Review from Harbor Freight:
Just put a plate/shield on. They're $125 each and will be the best deterrent.
I'm too cheap for that so I welded cables I had laying around to the cat and to other places on the body.
the whole idea is to deter. Stickers might. But a plate across the cat or cables securing it to the car are most likely enough.
they can be defeated but an unprotected Prius is a known thing - cut here, cut here and 2 minutes later you're just taillights.
they will just move on to the next one.
