Korean transit visa required for Indians?

You will need a B2 (tourist/transit) visa from South Korea unless you participate in the Transit Tourism Program.

Authoritative links on the matter seem to be highly unstable, but the Foreigners page at the HiKorea government portal states that visa-free entry is provided only to nationals of countries with visa waiver agreements with South Korea (India is not included) and those in transit with direct flights to certain rich countries (Indonesia is not included).

In addition, however, visa-free entry is also available to passengers holding onward tickets departing within 72 hours who participate in the Transit Tourism Program, a series of officially sanctioned English-language escorted tours. The tours are billed as free, but require a transit tour reservation, and may require a 10,000 KRW (~10 USD) fee covering the entrance to some historical sites. Also, the tours only operate at certain times of the day, so you will not qualify for this program, for example, if your arrival and departure times do not accommodate a tour in between.

You would need to present your tour reservation and onward flight information at immigration. From accounts, there is an information desk you can visit before immigration processing; however, you might want to make reservations beforehand, as it seems some tours may fill up. You will need to allot time to go through exit control and security screening before boarding your flight to Indonesia.

Visa on arrival, according to SkyTeam's TravelDoc website, is available

for visitors holding a completed application form received from a Korean embassy and one passport sized photo under the Frequent Visitors Policy.

To be eligible for this Policy passengers must have visited Korea four times in a period of two years or ten times overall, with one of these visits being in the last two years. They must also have no Immigration Offences or criminal record.

Visa is not required for passengers holding an Electronic Visa Issuance Confirmation issued by the Ministry of Justice of the Republic of Korea, which confirms the passenger holds an e-visa.