The thing that strikes me is that she used American date notation on the "coded" note -- AFAIK no Europeans put the month first. Probably a red herring though ;)
That's not the case here. The only way America differs from Europe in writing the date is when you include the year. She didn't do that on the note, so whether it's "October 22" or "22nd of October" carries little meaning. I'd go for the shorter O22 myself as well.