If there's physical access the data can always be accessed. Even encrypted data although it takes a bit more effort.
Yes, the bios can be resetted but that will also reset the password in it so you would know if you check it later.
Some stores sell clamps you can put around the box so it can't be opened without a grinder or some such, again quite easy to tell
The only way to keep your data 99% safe is to have it on a removable media, put it in a safe and sink it in the deepest part of an ocean, although I've seen stuff on x-files that would be able to fetch it.