I have observed this also. The Sectopod will sometimes just shell the area where you were spotted by some other alien. It even happened to me that I had already moved my guys away and the barrage hit the region my squad had been standing in the last turn.
But I have the feeling that the Sectopods are a bit bugged (or maybe used to be up until the last update), with their positioning and visibility and also with being able to target them (sometimes they seem to hover between layers when they are standing on a roof for example), so whether it is a bug or a feature I can't tell!
I have the impression that Sectopods can't be taken out by an overwatch trap easily exactly because they can hit you without being in your visibility range.