The Hindus, like the Greeks, usually had no mark for addition, except that “yu” was used in the Bakhshali manuscript Arithmetic (which probably dates to the third or fourth century).That's interesting. The Silk Road (http://en.wikipedia.org/wiki/Silk_Road) was in full swing at that time. This is pure guesswork, but multiple classical Chinese characters now pronounced yu (予,与,餘,逾,與) meant something close to addition. I wonder if there's a link?
There were a great many languages in operation there, and the Chinese were primarily interested in trade. It wouldn't surprise me at all if there were a regional patois language of business utilizing Chinese idiom that the subcontinent and others were able to borrow from.
Likewise, the borrowing may have gone in the other direction.
Note that some of the above symbols are not so dissimilar from the + symbol, allowing some minor scope for simplification.
On further investigation, http://www-history.mcs.st-and.ac.uk/PrintHT/Bakhshali_manusc... claims the Bakshali manuscript yu was "for yuta" (apparently Sanskrit/Devanagari: युत). Apparently yu is a well recognized word root meaning "to mix" or "(equipped) with" within Sanskrit, yuta being some kind of derived or more expanded/standalone form. (Sources: http://www.hindunet.org/hindu_history/sarasvati/dictionary/9... http://vedabase.net/y/yuta http://dictionary.tamilcube.com/sanskrit-dictionary.aspx?ter... http://www.hinduwebsite.com/sacredscripts/sanskrit_dictionar...)
That yu is apparently the same Sanskrit / proto Indo-European root as yoga, or even the far-western-Barbarian modern derivation in English, yoke. http://dictionary.reference.com/browse/yoke https://en.wiktionary.org/wiki/Appendix:List_of_Proto-Indo-E...
Hrrm... there must be something published on this already. Anyone with academic library access? :) I'm up in the Himalayan foothills, hehehe.