I would guess that your customer is exporting the faulty items or disposing of scrap items under a procedure similar to our Outward Processing Relief (OPR), the use of which will probably save them both import duty (if applicable) and GST costs. To control OPR they would have to have sight of the returned or replacement shipments and ensure they were correctly logged with Customs on arrival. This is only a guess – why don’t you ask them for a full explanation. In the meantime you could look at the very useful Singapore Customs website which does explain their Customs procedures (select from the home page “Traders”). Also, why not consider changing your delivery term for repairs/replacement to DAP arrival point and putting the onus on your customer’s company to get the goods through Customs therefore preventing you from being blamed for delays.