MySQL Handler_read_rnd_next == tablescans without indexes?

Question

I am in the process of tuning our Percona 5.5 server, and have been using percona's tools to plot our database's performance. In the MySQL Handlers Plot, i have been noticing alot of "Handler_read_rnd_next" calls.

After doing some digging in the mysql dev docu, this handler supposedly indicates: "this suggests that your tables are not properly indexed or that your queries are not written to take advantage of the indexes you have"

I then started analyzing our queries for non-indexed calls with the help of tcpdump and mk-query-digest (awesome tool btw) with the options:

• --filter '($event->{No_index_used} eq "Yes" || $event->{No_good_index_used} eq "Yes")'

but am drawing a blank: all the queries which are being listed as not using an index are ones which have a "derived" select type in them somewhere and all finish within a few milliseconds.

Hence my question: if the derived select types are the only ones without indexes, where are my Handler_read_rnd_next instances coming from? I'd appreciate any help i can get on trying to find out what the issue is.

Thanks!