Doing a quick findall in the files the only place i see 'plains' listed is your outskirtsarray in expansion.gd where you define it as
Line 77: var outskirtsarray = ['wimbornoutskirts','forest','plains','gornoutskirts','amberguardforest','frostfordoutskirts','shaliq','umbra']
otherwise it exists nowhere else in your code or in the main game code. So must be migrating from there to globals.state.offscreennpcs somehow.