Zzzap reading signals, snuck up on

“I checked out the information it was sending back to their base. Straight low-light video plus infrared imagery. Oh, right, yeah. And it listened in on a few walkie conversations. It had a good hour of watching us altogether.”
“Are you sure of this?”
"Pretty sure, yeah."
“Why did it take you so long to notice it?”
“Well, they are passive scans and it kept a really high altitude this time. There wasn’t much to hear until it was right on top of us.”

Excerpt From Ex-Patriots, pg 152-153
Peter Clines
https://books.apple.com/us/book/ex-patriots/id585738393