I had the same problem before too. At first I thought it was the seat sensor, but there is still ping noise if the passenger doesnt wear seat belt so thats not it. And after surfing through the internet, I found out that its either my IBS sensor is kaput or I need to register the battery to get it to work.
Went to Optimum, they refuse to believe my ideas and started cleaning all the terminals especially the one connected to the +ve side. After thorough cleaning (with WD40 no less lol) resetting the alarm and its gone for good. Apparently, the connector can become loose and triggering the alarm.
My advise, go to Optimum and get them to clean the connectors and reset the "restraint" alarm.