Indeed, that sign appears in one level, too...is it Fort Cavort? At least in some versions, I think. I have never figured out what it means, though for the longest time I thought it just said "SIGN" (guess I transliterated the first two letters and then stopped paying attention or something.)
I do recall seeing that sign, and figured it was a typo of SILO (because rockets). But I don't actually see it in the level maps on the Keen Wiki. Either it was in an earlier version or I saw it while browsing through the tileset at some point.
I'm at least 99% sure it was in Fort Cavort. Given how many years I played the version where that map was broken (which is not the one the KeenWiki displays) I suspect it is a version thing after all. In which case, I had no idea that would've changed between versions...anyway, I think "SILO" in that case would be a great theory for what it was meant to say, and it certainly wouldn't surprise me that a sign in the Keen 3 tileset would have a typo in it
When it was put in it's proper context, I assumed it said SILO by default, but my brain didn't know that mystery SIEO sign ever existed when Szemi brought it up.