Vietnamese speculators are stashing away bags and bags of robusta, a caffeine-rich bean used in instant coffee and blends.
The hoarding, the scale of which traders have never seen before, has cut exports and threatens to deepen a global deficit.
People in the market say that in recent weeks speculators - ranging from traders in fertilizer and petrol to government employees with idle funds - have been stocking up on beans whenever prices dip below psychologically key levels.
"When prices are below 40,000 dong ($2) per kg, purchases for speculation happen," said Do Ha Nam, deputy chairman of the Vietnam Coffee and Cocoa Association.
The hoarding may cut Vietnam's international robusta shipments in March for the second consecutive month, worsening a wider shortage.
In October-February, exports fell 11 percent from a year earlier to 8.94 million bags. Even if March exports match February loading, shipments in October-March would still be around a quarter less than a year prior.
The downtrend in exports could increase a global deficit that some estimates hold may be the deepest in almost a decade.
Domestic robusta prices climbed to a seven-week high of 41,100 dong per kg on Feb. 24. But speculators did not take the bait, reckoning the market could rise to 45,000 dong, following a disappointing 2014/2015 crop ravaged by dry weather and disease.
Traders say speculators may not release their stocks until late May or even June, when rains are expected to arrive in the Central Highlands coffee belt. Rains would bode well for the 2015/2016 crop, replenishing inventories and lowering prices. Speculators would have less incentive to hoard.
The smaller 2014/2015 harvest in Vietnam, on top of lower yields in Brazil and Indonesia, may lift benchmark ICE robusta futures to $2,115 a tonne by the end of calendar 2015, according to a Reuters poll in January.
That is more than 10 percent above current prices. But it is premature to say if a cup of instant coffee would cost more this time next year.
The hoarding in Vietnam has made it difficult to gauge the volume of beans left unsold in the country and forecast their impact on the wider market when the speculators release their stocks.
It is also too early to assess the size of the next crop due for harvest in October, traders say. ($1 = 21,320 dong)