I've found that if I leave a Garmin Connect tab open on my PC in Firefox for an extended period of time (say a day or longer, though sometimes shorter) that it randomly adds an excessive amount of water added, sometimes past 50L.
Last occurrence was today, though this time it had only added around 5L or so in the past 18 hours or so.
I haven't experience the phantom adding of water when I've ensured I've left no Garmin Connect tabs open.