It could be hashtags but without a #hashtag in the content. Could add another label next to the hashtag to mark it as a topic (could have multiple as well)
Are you doing this with hashtags or something else? If more than hashtag, please take a look at something I came up with this weekend for labeling posts…
https://github.com/s3x-jay/nostr-nips/blob/master/68.md
Once I get nostr:npub1wmr34t36fy03m8hvgl96zl3znndyzyaqhwmwdtshwmtkg03fetaqhjg240's feedback, we'll put it up as a PR. But the "unstructured labeling" section would handle the use case you're talking about. Would even let people label things after the fact if they forgot to label it when they posted it.
Discussion
So you’re using a `t` tag. That works, it just can’t be corrected after the fact if they don’t get the tagging right (which admittedly isn’t the end of the world or anything).