Most likely written down somewhere. The seed phrase is the backup method of storing a private key to a crypto wallet. You're supposed to put it somewhere safe as a way to recover the wallet if the normal way to access it (a software app or a hardware device) fails.
Brute-forcing a full 12 or 24 word phrase would take centuries to millennia, so there's only a few possibilities:
- They just found the full phrase written on a card in a safe somewhere, in which "deciphering" it is as simple as typing it into a fucking wallet app;
- He was smart enough to split the phrase up and keep different parts of it in different places, so they might have had to brute-force part of it;
- They found a hardware wallet and hacked into it to recover the phrase;
- (exceedingly unlikely) they figured out that the random number generator he used to generate the phrase was broken and had predictable output patterns.
Either the bowl or the shroud around it is probably overstimulating the poor guy's whiskers.