After rethinking about this implementation I guess entering a filter with ':' is only be applicable to the account hierarchy and not to any other split or transaction detail.
It would probably be better to implement this as shown by the following pseudo code:

for iterating through splits

  if (filter text contains(':')) {
     search for account hierarchy
 } else {
     search in all other transaction details
}

Can anyone confirm ?

What if "MyMoneyAccount current = acc;" is not the standard account from the beginning?

You should use MyMoneyFile::AccountSeparator instead of the colon character. It is used in all other locations.

Isn't the question what would happen if acc is a standard account from the beginning more interesting here? I don't know if that could happen, but usually if called with acc being an account somewhere in the hierarchy going upward always ends with a standard account sometime. If not, we have a real problem.

Use MyMoneyFile::AccountSeparator here also.

Not sure, but I do have things like "EREF: xxxxx" in my memo field which I could search for.

Then it should be as currently implemented with this patch

added additional guard