I am at the Hilton in Narita, and I've just come back from a wonderful meal of Shabu-shabu. One of the dipping sauces was stated to be soy sauce, but it had a lovely sweet sort of flavor to it, so it clearly was more than just plain shoyu. It came with green onions and something pink that was stated to be spicy to mix into it. What was likely to have been in it? Is this something I can recreate at home?
There are a number or styles of soy sauces used in Japanese cuisine. Saishikomi shoyu is sometimes described as sweet compared to regular Koikuchi shoyu. Additionally there are soy-based sauces available in the US that have added ingredients such as corn syrup or MSG to enhance specific flavors.
Kikkoman makes an "enhanced" soy sauce that they call Amakuchi shoyu that is sweetened with corn syrup, as well as another call Umukuchi shoyu that has added MSG for savoriness. Teriyaki sauces, especially those made for use as a marinade, can be as liquid as standard soy sauce and are sweetened with sugar or corn syrup.
A traditional dipping sauce for shabu shabu is a 1:1:1 mixture of shoyu, dashi, and mirin reduced by 1/3 with rice vinegar added to taste. If you wanted it even sweeter, you could just add sugar to taste once it has reduced.