User blog comment:Miskos3/Episode 272: Fairy Tail Zerø: The One Who Teaches Magic, Review/@comment-4933386-20160221022412/@comment-7418318-20160221083748

Zeref was clearly panicked, which makes you overlook quite a few things you could otherwise tell.

Also the best illusions are those that blend so perfectly into their enviroment that they feel normal. Animals in the woods are normal (although not in that number), but an army of absolutely identical soldiers appearing out of thin air and standing in 100% perfect formation without moving a single muscle... that is suspicious to anyone who's not a total idiot.

Not to mention that Mavis was openly using magic, so Geoffrey didn't have to wonder wether it's magic or not, he "only" had to determin what kind of magic it was. While Zeref didn't even know was a Wizard at this point, adding to the surprise.

In addition I suspect that summoning actual soldiers would requite a lot more magic power and thus could be felt by talented (or perceptive) wizards. If true the lack of Mavis emitting strong magic power could also help tip Geoffrey off to the nature of the spell. Though that last paragraph is just speculation on my part.