Went to bed with a $10 budget alert. Woke up to $25,672 in debt to Google Cloud

a master key can use any lock. there is no reason why such things should not exist. u know. for support. Otherwise, through bugs. a less paranoid view:

MS had a similar one on azure at some point where you could get some 'master key' to access _all tenants_ (it wasnt an actual key but the effect was identical to what you observed. non existing key with all access. (luckily in that specific case it was a benign researcher)