Because seals are unique to the user. It will say "So and So's Seal."
If someone broke it then resealed it then they could only make it look innocuous if they had somehow acquired the sealer's signet ring.
It'd also be no different if automated.
Eg.
With automation:
1. I seal my letter.
2. Someone opens the letter.
3. Letter is marked as having a broken seal.
Without automation:
1. I add the line, then seal my letter.
2. Someone opens the letter.
3. Letter is marked as having a broken seal.
The only condition where automation might be functionally different is if it somehow noted when it had been resealed with automation, which is unnecessary because seals are unique.
This is correct. It's possible to unseal a letter in real-life in such a manner you wouldn't know it was previously sealed. Sealed letters only work under the pretense you are leaving a partially broken seal there; And that's not even digging into the mechanics of people with sleight-of-hand managing to effectively re-seal a document using a localized heating source on the back of the seal.
Seal tampering, and seal tampering methods were a big deal back in the day. If you want to ensure that your letter is known to be sealed, it should have a footnote denoting that the letter should have arrived sealed. In theory, you can emote at the top of a letter yourself, when writing it, stating that a broken seal is present (as it would have to be in order to open the letter and read the line) thus informing the reader it was once a sealed letter, whether they opened it or not, which is as Sol suggests.
However, emoting that the letter has a broken seal could be considered foul play. It would be better to simply note in the writing of the letter, that it should have arrived to the recipient sealed. To emote the seal being present, despite the fact seals could be removed by skillful hands, or even re-sealed by skillful hands, is cheesing around a system that operates in a binary state. Sealed, or open letter. There is no mechanical representation for partial or broken seals.
So my advice? Just write:
"John,
This letter should have arrived to you with my seal in-tact. Go to the farm, and get the kids and meet me in the city in two hours. Don't tell anyone that we're leaving, or where we're going. It's safer that way.
Your Wife,
Sarah"
