Passing the teller a note is intended to always succeed in normal security and always fail in high security. The bank drops high security much more slowly than other locations, and $5,000 every couple months doesn't make a huge dent in the economy of the game. Plus, you don't get any skills from it, and bank robbery is a pretty serious crime in-game, so I felt okay with it always working on low security. Unfortunately, it seems that sometimes the bank doesn't go into high security afterwards; not just from this thread, I've seen more than one report of being able to chain that tactic.
It's intended to be really easy the first time, telegraph that it might fail a second try by setting the site to high security, and then punish you if you try again anyway.
There's more room for change here -- the current implementation doesn't present any risk of the teller sabotaging your money with a
dye pack, which is designed to be a low-key countermeasure to exactly this style of low-key robbery. But I didn't ever come up with a really good way of making that interesting from a gameplay perspective; is it just risk? Based on security level? Security level makes sense, but I figured security guards would be a more interesting failure than your money being destroyed after you leave.
Another option from a development point of view is to have a chance that police chase you after you leave the site, even if you just slipped a note, and especially on high security.