Why would you care about an empty address getting compromised ? You won't be able to know who have  your private keys unless the address has balance which is in this case ... the hacker will take the funds so you will know .
The strategy you used isn't the best, though , It's only going to increase the chances of you getting hacked and you will have to check every device if it contains viruses or something.

What I would do is print the private keys on a piece of paper (two copies max) and store them very well in two different places where no one can reach them.

+1, it's even completely possible a hacker has your private key, but doesn't use it untill enough inputs have been created for this address, so even transferring a small amount of coins to the address to test out if the keys have been compromised will do you no good.

My advice: never use this private key, generate a new one in a secure environment (offline pc, paper wallet, hardware wallet) and use that one instead

um.. so you have a private key on SEVERAL different drives.. some of which you have NO IDEA where they went.
i assume these keys are in plain text?

how certain are you that your old pc, your flash drive, your old phone, and your external drive are still around?
if there is any chance at all someone has access to that.. i would not use that bitcoin address.. just make a new one.

I think that's the best move forward, but then all of my devices are in the house. I was concerned software-wise.

How did you get that address ? VanityGen from a trusted source or online websites .If you have generated that address using https://bitcoinvanitygen.com/ or similar services then most likely they will store your private keys which can be used to steal your funds.

That is a bad way of storing a private key.You could choose one and make it as secure as possible.In case of back up,another alternative method such a paper wallet could be added.There is no way to track if your private key is compromised until you see some significant changes in your wallet ,for example : Your wallet is empty after you transferred funds.

1. No, I used samr7's binary file that I got from his repo: https://github.com/samr7/vanitygen/downloads
2. I agree. That's pretty significant.

I'm generating a new key at the moment

If you think that your private key could be compromised, generate a new address that will have a new private key, and keep it on a secure place. And don't store it on phone, or cloud services.

what is going on here? why did you even save your private key on this many places at first?
besides you should store your private keys in encrypted format or at least put it in a .rar/.zip file and password protect it not just putting it on a .txt so anybody can read!

and since you say it is empty, just create a new bitcoin address from a brand new wallet to start fresh and be sure you are safe.
it is best to spend time and become safe now rather than being always suspicious.

Update: I just loaded my address with more or less $20 in BTC and it didn't get stolen in 30 seconds. I left a small amount of funds on it just so and transferred the rest to my new address.

I think it's safe to say the address I mentioned isn't compromised, but I still have to track down all of the encrypted copies of my private key. I think there's one in my mom's phone.
I've successfully tracked down all unencrypted copies of my private key.

In the end, this story had a good ending.

So why are you so skeptical as if, you are new to doing stuff on computer through the web?

Just be optimistic, be incharge, on your own part ensure to implement the protective as suggested by other member's.

Final Note : be less assured that NO ONE CAN GUARANTEE 100% SAFE ONLINE else microsoft would'nt have abandone windows XP operating system to hacker's.

Regard's
Arlene