A SHADY deal involving worthless Argentinian pesos being offered for sale in a Hong Kong hotel has baffled the territory's detectives. And the very presence of the South American currency has got foreign exchange dealers wondering what the pesos could be doing in Hong Kong at all. Five people have been arrested so far and Commercial Crime Bureau detectives have not ruled out more, but in the meantime, they find themselves with 195,000 pesos which became worthless in 1984 when the currency was devalued. Chief Inspector Matthew Hemmings said this type of investigation was fairly new to police and consideration was being given to using Interpol to liaise with Argentine authorities. The investigation began when two Hong Kong men were arrested in their hotel room after allegedly trying to sell some of the notes, which were now non-existent 1000-peso denominations. A total of 100,000 pesos was seized from their room. Further enquiries resulted in the arrest of three others - a Hong Kong man, a Filipino man and woman in Kowloon. A further 95,000 pesos were seized. In 1984, with the devaluation of the currency, a 1000-peso note could have been converted to a one-peso note. One peso on today's exchange rates is the equivalent of about HK$7.7. Foreign exchange dealers contacted by the South China Morning Post said they believed there was little or no trade in Argentinian currency in Hong Kong. ''It's just not useful in any market other than Argentina,'' one said. The five arrested have been released on police bail. Anyone with information about the currency should call the Commercial Crime Bureau on 582 5512.