It’s a little bit more complex unfortunately. If a retention policy is set up for deletion only and there’s no other form of legal hold enabled for a user, data will be permanently deleted from the hidden SubstrateFolder potentially much quicker than 30 days. Generally what happens is that after a message is marked to be deleted, it can take a few days before it is actually deleted and moved into the SubstrateFolder, from there it can take 1-7 days before the message is permanently deleted.
It is still worthwhile to check if messages can be found using a content search.