Jon is has been in the warehouse since quite a few updates. I think I recall that there was a short period where he was somewhere else, outside one of the houses I think, maybe Andrea's but I can't quite remember for sure.Game isn't finished, and as of the end of end of v0.97 Jon is still very much inside the warehouse, as is Erica.
But he has always been available since he was introduced, most of the time in the warehouse.