If they really wanted to befuddle outside forces, they would have had the whole door warded. Three is a minuscule amount.

[Berna follows Thomas glancing back and forth, not particularly worried about their backs (several more crystals there). She nods when he instructs her to keep the door unlocked.]

I'll stay near the entrance and you go look. Ah, but before you go...

[She waves one of her crystals over and has it fly closer to Thomas.]

This one will follow you. Only for this job though. Unfortunately, you can't keep it. [She grins waving him off.]