No experience with the Chummie but had great success with the Malem. I remember the Malem instructions saying to start using it again if night wetting starts to re-occur and I recall doing that once or twice after the initial "discontinue use after 2 solid weeks of dryness" and it working with just 1 or 2 "reminder" days.
I think the sensors are made to be highly sensitive so yes, it could be reacting to just a drop or two of urine in order to trigger your DS to wake up when that feeling happens. Maybe if it's going off with a drop or two and your DS isn't visibly wet that's a good sign that he's holding it?
Mom to two amazing DDs ('07 & '09) and a fur baby.
Gluten free since Nov '11 after non-celiac gluten sensitive diagnosis. Have had great improvement or total elimination of: migraines, bloating/distention, heartburn, cystic acne, canker sores, bleeding gums, eczema on elbows, dry skin and scalp, muscle cramps, PMS, hair loss, heart palpitations, fatigue. I'm amazed.