From the documentation:
<F5> - Refresh the match window and purge the cache for the current directory. - Remove deleted files from MRU list.
This assumes you're in ctrl-p mode already. Note that you can hit F5 in the middle of a query, i.e., you can type a few characters, find it's not matching a recently updated file, and hit F5 to refresh right then. It will automatically show you the match if the file was just added to the ctrl-p cache.