Search is really good in Obsidian, so anything unique works. I’ve used tkk and kakunin (Japanese for review, confirm) dropped randomly in note to remind me to go back and check something.
I also use single brackets for words or phrases that could be omitted. e.g.
..text someone at the last minute [if you were running late].
or for the best option to choose later:
This meant that [you / people] always had to...
You can combine them: [tkk-railway-name-at the-time].
An added bonus using single brackets [...]; they are considered “bare links” and you can target them with CSS to change the styling of the text if you’d like.