Travel Insurance for Singaporean working overseas on short period?

Yes, you are right that most travel insurance policies provides coverage for trips that depart and return to Singapore.

To add on, you can travel between countries in between the travel period, e.g. I buy a travel insurance that covers worldwide. Accordingly, my trip was to travel from Singapore to Germany, then to Switzerland, before returning to Singapore. In this case, travel insurance covers for the entire trip.

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Presumably the country you are posted in has similar products and you can get a policy there? It is fairly standard market practice to insist that departure and return is from the country is issued. Anything without that requirement would be bespoke (ie. Expensive!)