Toad's "object search" routine will look through ALL_SOURCE (and other parts of the data dictionary). Of course, this will be limited to the objects the connected user is allowed to see. If you have access to the DBA version, great. If not, you won't be searching everything.
Also, if you're way back on Oracle 7, or your database was migrated up from Oracle 7 to 8i or 9i (not sure about 10 or 11), then trigger source may not appear in the user_source or all_source views. Best to check. The easiest way I've found to get it to appear is to do an actual modification -- add a space, for example -- and recompile the trigger.