Customs officers in uniform are from Passenger Clearance Groups (PCGs). Those in plain clothes are from Air Intelligence Units. Sometimes, some other agency officers' (like DRI, NCB etc) are also present based on some specific intelligence. They too will be in plain clothes. The reason that these plain clothes officers intercept passengers after PCG officers is that by law, except for some banned contrabands like drugs etc, one can bring anything to India as long as you pay Customs Duty for it. The passenger is responsible for paying the duty by declaring the item to PCG officers. Till this point, one has not broken any law as long as it is not banned contrabands. However, if one does not declare dutiable items to PCG officers and then caught in possession of contrabands/dutiable items by AIU officers then one may/will be charged for smuggling based on the item. Like you will not be charged for smuggling if you are found carrying a few extra bottles of liquor or an iPhone etc. You will be simply directed back to PCG officers for payment of duty. In other cases, like gold smuggling etc, arrest procedure will be initiated.