Chances are, there's water leakage.
Do you use RO water filter? The machine often throw away water in the tank and replaces it. Once I measured, it filled an entire bucket from midnight to morning.

Anything that uses a floating ball may be compromised and causing water to overflow. Water tank, toilet cistern, etc.

leaking is possible but TS mentioned that already >20 years with > RM100 water bill

how can a leaking last long for 20 years with no one detected and caused problem at his/her house and nearby ?

3 ppl can go rm100+ conlan7firm leaking la.

100000% leaking.

Apparently this world damn a lot of sei sohai dont know how to check leaking or not. Speaking from experience. I had tenant swear keep say no leak but bill rm100 to rm200 then up to rm300. I had to hire plumber go check for him. Straight show me tangki already leaking non stop, water stain berlambak. The tenant reaction? "Ohhh ya ke, ini kira leaking ye? "

TS u are definitely one of those sei sohai. Hire a plumber go check