Star Trek's [universal translator](http://en.memory-alpha.org/wiki/Universal_translator) is always either directly depicted as a device (see images on linked article) or is implied to be a device somewhere on the speaker's person (most likely built into Starfleet com-badges).

But there are many instances where:

1. Characters imprisoned after all their equipment and uniforms are taken away continue to communicate with aliens who don't speak English or don't have their own UTs.
2. A character forgets (or is made to forget) his identity and lives among an alien race that speaks another language.
3. Characters communicate with alien species in a location where dampening fields or other phenomena render technology ineffective.

Putting aside number #3 (which is a killer), does this mean that the most likely possibility is that the UT is an implant? Canon answers not essential (I couldn't find any myself), but alternative possibilities are welcome.