I have always played it the Ambush marker. We say on page 142 that "ambushes are made by placing and mabush marker" - so its the marker that is the ambush, once the troops go on it is no longer an ambush if you see what I mean.The original question was about the wording on p 142 "Ambushes must not be visible from the enemy deployment area" and whether that refers to just the ambush marker itself, or the positions of the individual bases when they actually get placed on the table. Any chance of an author-agreed answer on this?
We don't like any memory or forward thinking effects in the rules and you don't know where the actual bases are until later. In fact in line with the philosophy above they are quite likely moving about in secret in response to enemy movement rather than bolted to the ground.
The rules are pretty restrictive later on where things can go. For example, imagine you hid a marker in a minimum size wood but it was a 12BG of warband that technically wouldn't fit (they might but imagine it anyway). Well if there are no enemy so when they deploy they can still do so "unseen and further away" they get away with it. But if anyone is in sight of where they would need to spill out they could not ever legally deploy. They would then either have to stay cramped in the wood and wait, or if forced to deploy would lose any bases that could not go down (probably killing them off in the process). Thus "deployer beware" is a decent motto.
Not an official answer yet from the 3 of us, but would be fairly surprised if RBS or TS disagreed, so hope that helps thinmgs along.
Happy to prompt a look from them to formalise if needed.