Possibly the Harappans and early Thais were trading partners;Indus-type seals have been found both north and west of the Indus sites.
Harappan beads can be identified as to location of manufacture (that’s someone’s idea of a dream job, right there), and have been used to track trade routes all over Asia. They got around. The Sumerians’ own origin tales involve arrival by sea (in what is today the Persian Gulf or to some Shatt al Arab); the names they used for most of their great cities and the rivers are not Sumerian names, meaning they were borrowed from whomever lived there before the Sumerians took over. They had seagoing trade networks, and I would probably not be the first to suggest they came from the Indus Valley.