Steps to reproduce:
0) Sign in with an account which is not exempt from the search flood limit
1) Set $acm_type in ACP to 'null';
2) Make any keyword search that has more than one page of results
3) Receive error:
"Sorry but you cannot use search at this time. Please try again in a few minutes."

This is because the user_last_search column in the users table is updated if the search results are NOT retrieved from the cache. This is the case on every page load with acm_null. This is not the case with the other search plugins because all results are stored in the cache at page one and subsequent pages are retrieved