So, I grew up in an abusive home and sometimes advise people trying to get out of bad situations.

The "secret" part actually isn't because someone's just naturally up to no good. There's actually a pragmatic, "we're in a survival situation now and this is how you survive" reason.

Keeping the bug-out account secret is because when shit gets bad, like, bad enough to need to get out ASAP with the shirt on your back, other people even knowing you have the escape money can cause them to weaponize that knowledge on you to manipulate you. Like, "You give me that $500 you have, or I'll kill your cat."

Or they can try to log into your account to see stuff. Like, if you did successfully move out, they can try to log in and see if your address changed, and what it was changed to, so they can stalk you.

Family members know things like past addresses, social security numbers, mother's maiden name, so if you don't fudge those questions and such and they're aware you have a bank account at so-and-so bank, they can sometimes get past security measures by pretending to be you since they know your intimate information already.

Keeping the entire bank account secret is one more layer of defense so they don't know to start trying to get into it, and don't know to weaponize their knowledge of it on you.