user117191: I'm new to Bitcoin and trying to understand it, step by step. I tried to make a wallet using Electrum. This gave me 12 words which seem to allow me to regenerate my wallet's private key. When trying to use this 12 words in Exodus (another wallet app) it says those 12 words are invalid. So, I figured these 12 words might be an app internal thing...
Seeing Chris Chen's comment here seems to confirm this.
So, the question would be: if I happen to get my 12-words compromised and someone gets to know them, would I then be still safe as long as this person does not figure out which wallet app I used?

user112077: 
if I happen to get my 12-words compromised and someone gets to know them, then I would be still safe as long as this person does not figure out which wallet app I used?

Technically, no. The attacker only needs to figure out the algorithm used to convert the passphrase into the public key, which may be used by multiple apps.
But that is probably not what you are asking. To answer your real question, if your wallet app is popular, or uses the same algorithm as a popular app (and presumably new Bitcoin users tend to use popular wallet apps), then it would be extremely easy for the attacker to figure out which algorithm you used and therefore compromise your private key. They would simply need to try every popular app, as m1xolyd1an pointed out, and there so few of these that this task would be trivial. So no, you would not be safe.
(This is theoretically the same as trying every possible passphrase, except that there are so many possible passphrases that this is basically impossible.)

user26873: No your funds would still be at risk. There are only a few popular wallets that use seed phrases.
Most wallets use BIP39(https://en.bitcoin.it/wiki/Seed_phrase) but there are some that use different versions or no seed words at all.
Electrum does not ouse BIP39, see their docs explaining why. https://electrum.readthedocs.io/en/latest/seedphrase.html
It would be extremely easy for an attacker to try the most popular versions and steal your funds.

117191: <p>I'm new to Bitcoin and trying to understand it, step by step. I tried to make a wallet using Electrum. This gave me 12 words which seem to allow me to regenerate my wallet's private key. When trying to use this 12 words in Exodus (another wallet app) it says those 12 words are invalid. So, I figured these 12 words might be an app internal thing...</p>
<p>Seeing Chris Chen's comment <a href="https://bitcoin.stackexchange.com/questions/85385/have-12-word-phrase-need-to-find-wallet">here</a> seems to confirm this.</p>
<p>So, the question would be: if I happen to get my 12-words compromised and someone gets to know them, would I then be still safe as long as this person does not figure out which wallet app I used?</p>
 112077: <blockquote>
<p>if I happen to get my 12-words compromised and someone gets to know them, then I would be still safe as long as this person does not figure out which wallet app I used?</p>
</blockquote>
<p><em>Technically</em>, no. The attacker only needs to figure out the algorithm used to convert the passphrase into the public key, which may be used by multiple apps.</p>
<p>But that is probably not what you are asking. To answer your real question, if your wallet app is popular, or uses the same algorithm as a popular app (and presumably new Bitcoin users tend to use popular wallet apps), then it would be extremely easy for the attacker to figure out which algorithm you used and therefore compromise your private key. They would simply need to try every popular app, as <a href="https://bitcoin.stackexchange.com/a/103541/112077">m1xolyd1an pointed out</a>, and there so few of these that this task would be trivial. So <strong>no, you would not be safe</strong>.</p>
<p>(This is theoretically the same as trying every possible <em>passphrase</em>, except that there are so many possible passphrases that this is basically impossible.)</p>
 26873: <p>No your funds would still be at risk. There are only a few popular wallets that use seed phrases.</p>
<p>Most wallets use BIP39(<a href="https://en.bitcoin.it/wiki/Seed_phrase" rel="noreferrer">https://en.bitcoin.it/wiki/Seed_phrase</a>) but there are some that use different versions or no seed words at all.</p>
<p>Electrum does not ouse BIP39, see their docs explaining why. <a href="https://electrum.readthedocs.io/en/latest/seedphrase.html" rel="noreferrer">https://electrum.readthedocs.io/en/latest/seedphrase.html</a></p>
<p>It would be extremely easy for an attacker to try the most popular versions and steal your funds.</p>
