FYI the airport is under MECA even though it is technically in Hendricks Co.
The existing terminal and both runways are located entirely in Marion County. The new terminal is located entirely in Marion County. The new interchange for the new terminal is located almost entirely in Marion County.
Take a look at a satellite map of the area and draw an imaginary line due south from the intersection of U.S. 40 and Raceway Rd. That line defines the border between Marion County and Hendricks County. You will see the perimeter road that runs south from Bridgeport (which is in Marion County) past the former United Airlines maintenance complex. That perimeter road remains to the east of the imaginary line you extended south from Raceway Road. All of the airport is also east of that perimeter road.
Hendricks Co uses both SAFET and MECA.
For the most part, Hendricks County public safety agencies only use MECA when they are dispatched on a run in Marion County. Otherwise, all Hendricks County public safety comms are on SAFE-T (with the exception of some car-to-car chit-chat on their old VHF frequencies).
To the best of my knowledge, the former MECA talkgroups for Plainfield and Avon were deactivated long ago. That's a moot point now that MECA has gone to their digital system. The frequencies that were licensed to Plainfield that were being used for MECA in Hendricks County have been transferred to SAFE-T. Hendricks County dispatchers have access to the MECA CAD so they can see what Marion County units might be available for mutual aid and they have MECA talkgroups in their consoles so they can dispatch Marion County units on Hendricks County runs.