He wanted to spare them poor souls.
They were probably long gone after seeing what he did to those other ninja and Konkuro talking about the scroll gave them the perfect amount of get away time.
He probably didn't actually sense the three, and was talking about in a general sense.
Team Kunerai probably thought Gaara would hunt them down next and forgot about Kankuro
Kankuro thought Gaara would execute him next and forgot about Team Kunerai (if he even noticed them)
Meanwhile Gaara's all "bring me things to kill"
Nope they were still there Gaara didn't kill them but just a cover for his gourd see :
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The plot shield was stronger than gaara