This is a new one to me. Today I got this pre-arm failure while sitting on the table in my dining room. Mission Planner was not connected, so it would have no source of terrain data, which is understandable. But why is this a pre-arm failure? Is this only a pre-arm failure if TERRAIN_FOLLOW is enabled?
I’ve always had terrain follow enabled and never before had this pre-arm failure.
Parameters were all enabled for terrain.
TERRAIN_ENABLE = 1
TERRAIN_FOLLOW = 1
TERRAIN_SPACING = 30
Mmm… I test for first time my quadplane and an advertise in Hud says NO TERRAIN DATA the hole test, I asociate with something that I don´t calibrate yet but now that you mention, perhaps something MP related?
I think that probably your GPS was off (because it was indoors) so it wanted terrain data from a bit further away than it had preloaded.
I had a quick look at the prearm checks and as you expected, it’s only a pre-arm failure is TERRAIN_FOLLOW is set to 1.
prearm check: https://github.com/ArduPilot/ardupilot/blob/master/ArduCopter/AP_Arming.cpp#L475
terrain_use: https://github.com/ArduPilot/ardupilot/blob/master/ArduCopter/terrain.cpp#L34
According to our terrain following page, setting terrain-follow will mean the RTL will climb over hills. I haven’t tested that recently but it might be so! 
http://ardupilot.org/copter/docs/terrain-following.html#using-terrain-altitude-during-rtl-and-land
I must have just gotten lucky in that I had terrain data loaded for everywhere I fly just from tinkering with Mission Planner at home.