I would suggest its the search function within WhatsApp. On Android if you open your conversation view, at the top is a Magnifying Glass (search function).
Typing words in there brings up messages (from all current chats) which contain the the words you searched.
Therefore any chats deleted after the search will have the searched items referenced in the ChatSearch V3 table. So not the full conversation but only messages containing the searched items.
Hope this helps a little.
I'm currently running some test to confirm this.
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