I couldn't see very well, but in one scene Fowler's helicopter was hovering near some explosions and the Nemesis. I couldn't tell if the explosions were coming from the Nemesis or not, though. Did anyone else see this?
Holy crap guise you know what that means? That means we're getting a new Decepticon!!1! EDIT: F**k! Sort of ninja'd.
In that they're both helicopters, yes. The thing is, Fowler's chopper is a different model to Airachnid; TFWiki describes it as being similar to a UH-1 Huey, whereas Airachnid's alternate form is similar to a Comanche. Basically, Airachnid scanned one make of chopper, but came out as a different kind of chopper.
Yes, many have seen the helicopter and I won't assume its Fowler's helicopter just yet. But let's say it is, the base alarm could have be triggered and alerted Fowler to investigate.