I have a Castlegarden XDC HD 150. My neighbour stupidly cut the grass when wet when I was off on business for a few weeks, and didn't clean out the chute.
The acoustic alarm indicating the bag is full remains constantly on despite a good clean out.
I'm thinking maybe some mud has stuck over the bag full sensor and it's stuck on, but I've no clue where the sensor is or what it looks like.