If I am not wrong, there are websites that have some private keys being shared or posted because I tried accessing one of it (bnb network) and I tried sending a very small amount of bnb to the wallet address that didn't even reach 1 dollar but once it was in the wallet ehen the bnb was transferred right away. Although I have seen it myself but I never knew what they did use or how they monitor the wallet but it is true that leaked private keys are being monitored and in btc, the same thing goes as well.
There are bots that I believe are just built for such activity. I have encountered one myself with a friend's wallet, which got compromised due to his carelessness running after airdrops. Once you make any single deposit to withdraw whatever you have in that wallet, the bot will automatically move the funds over to another wallet, and in most cases all the transactions are sent to just one wallet.
Recently, there is one new but old pattern of scam where the scammer will post their seed phrase online with a good amount of holding on them, and when you try to send a gas fee on whatever network the fund is located, it's either automatically moved out or you can't even move it out yourself because you don't have the complete access key to carry out such a transaction.