Beeping SmarTAG

Anyone know why my SmarTAG keeps beeping after I go through the toll?


There's the acknowledgement beep of "yes, taken your money, you may pass", then immediately followed by multiple beeps (about 7) of... what?

NOT:
- low money (beeped when more than rm50)
- low battery (beeped after trying a different batt)


Any ideas, or contacts I could ask?