At https://dev.twitter.com/docs/using-search search_metadata we’re suggested to use max_id from search response if no results are returned:
“Include a since_id when asking for Tweets. since_id should be set to the value of the last Tweet you received or the max_id from the Search API response. If the since_id you provide is older than the index allows, it will be updated to the oldest since_id available.”
I think you mean the max_id inside the search_metadata: next_results field.
As there is a max_id field, this may lead to confusion.