Takarashi
The idea is that Kreiva was attempting to use a mental defense technique, stilling her mind and focusing on an object that would make it seem like her presence simply 'disappeared.' And yeah, from Kreiva's perspective, sending any kind of signal would give away her position. She had no real reason to trust Silivene, given that the dragon just slipped into her mind and picked up on the mask, which was what was giving the girl so much panic to begin with. I'll reply once I wrangle some creative spark.
That's the thing, from Silivene's perspective there is no mental attack, just projecting her thoughts to someone else, like talking but mind to mind instead of voice to ear. The signal Silivene was asking for was about the only way Silivene would know that she was heard. She won't even be able to tell if Kreiva is blocking her out.
About the mask, I was just guessing and I hope I didn't get it wrong. But if it is magical or enchanted then Silivene would have noticed it. I've been assuming it was, so if I am wrong, I'm sorry I've been meaning to ask and haven't. If the mask isn't magical at all, then Silivene could still figure out that there was something that Kreiva was hiding. Kreiva was trying to get them to leave with a story that Silivene knew was false to begin with and wasn't trying to just ask them to leave and explain why. Kreiva then used her magic to try to fool them into leaving, so Silivene knew it wasn't her magic and most likely not herself she was hiding. As well it was not the lake she was protecting because the way she was using the lake for her trick, if her target didn't leave it would make them more interested in the lake. Therefore, most likely something she carried she was protecting.
Silivene's sensitivity to magic and her ability understand it come from years of experience with it and amplified by her bond with Malik. This is the thing that will improve as their synchronicity raises.
I hope this explains where my characters are coming from, these have been the thoughts going through my mind about this. The only time in character that these thoughts would have come out would be if Silivene had explained them to Malik, and I hadn't considered including them as part of Silivene's thoughts, I've been mainly focusing on Malik's point of view.