However, they can also be a double-edged sword since your occasional large investment activity may also trigger a SAR, especially if you're trying very hard to avoid triggering one, like by breaking up large deposits into multiple smaller ones. It's always better to just tell your bank what's going on so that you don't trigger a SAR on yourself.
For example, if you've just sold a car or a house and have a considerable amount of cash to deposit, this can raise a red flag, but it doesn't have to be if you simply explain the origin of the money. The same can go the other way, if you're trying to take a big chunk of your savings out of your account -- full disclosure is the way to go.