I answered a question about Roman contact with the Chinese kind of recently here

I haven't heard about Roman coins found in Cambodia specifically but I can tell you they Roman coins are found all over Eurasia. The ancient world, especially the classical world, was crisscrossed by trade. Goods would come from the Chinese and end up in Rome and vice versa. The only thing about that is that it wasn't Chinese traders bringing those goods all that way. Basically trade worked through an extensive network of middle men. The Parthians and Sassanids flourished when much of their land was desert by buying goods from India and the far east, moving them closer to Rome until a merchant would take the exotic goods at a greatly inflated price, bring them back to Rome, and sell those super inflated priced goods at an even higher price and get away with because Roman Patricians would pay for it. Tonnes of gold, yes the unit of weight, made its way outside of the empire and into the East but most notably it went to India for their silk. When Rome got ahold of its own Red Sea port the amount of gold flowing out and silk coming in was astounding. From India those coins could have ended up anywhere; maybe on a boat bound for a port in Vietnam then overland to Cambodia.

Sure!

Roman was already trading w/ India for all its spices and gems when the Parthians established their empire and their influence in Central Aais. One of the things they immediately did was to throw the Romans out and take over the lucrative trades in eastern luxury items.

Probably one of the reasons Rome had it for the Parthians and thus their long-term ongoing war.

One interesting thing about this early Globalization dynamics was the expenditure by the Roman Empire was that 1/3 of expenditures was trade w/ the east. One of the observations about trade between the West and the East is that the East has always had things the West wants and not the reverse. A historic trade imbalance since the Roman Empire!

Since Rome had no way to change this economic imbalance, the Roman Govt did the sensible thing: It established controls on imports on eastern goods and levied heavy taxes. No longer having a foothold in India, Rome instead established bases in the Red Sea and What is now Jordan to control caravan trade routes.

One area that they were able to make up for the trade imbalance was they had the technology to dye imported silk. Apparently Silk was so common in China that they would export it untreated/unprocessed. The Romans w/ their knowledge of dyes would process the silk and send it right back to the East.

One last bit of trivia: Counterfeiting in the ancient world was wide-spread and almost universal. The one thing that the Indians craved from the Romans (besides Gold, Silver, and Wine (!)) was Roman currency. Seems you could trust Roman coinage more than anybody else's !