[@ManyThings]Probably but they're mixed hidden by a special sort of magic, and magecraft. Monica just canceled out the magecraft section. She sensed the things by way of her second to last spell. While it is fair to say that out in the open you'd have seen them coming in the forest less so since it is much more in the domain of the enemy caster who's actively assisting the already potent illusion magic. I can't really say more without giving unintentional hints of enemy caster's identity. 

Point I'm trying to make the full body illusion gives them an equivalent of making their bodies invisible. While the other magecraft is a sort of presence concealment which will get stronger the closer they are to the proximity of enemy Caster. Regardless their strength isn't meant to be all powerful more to be useful killing tools Lancelot clearly showed that one can dispatched them relatively easily but that doesn't make them less dangerous.