Most of the external "antennas" have active circuitry on them, and they use a backup battery to keep the almanac and ephemeris data so the "first boot" time is kept short. Usually there is a similar internal battery in a GPS to keep waypoints and other data, so depending on what they have done, you can expect one or both to exist.
If you have a "waterproof" unit that can be more problematic, as some are gasketed but other are glued or welded shut. Which aids waterproofing but makes service
into a nightmare requiring a new case, or cracks and glue.
So it may be very different from one unit to the next. The actual battery, as you've found, can be the least of it all.