Sender ID: "Ayakashi", service no. 127-662236-C Signal origin: Sekiryu Recipient ID: Sanae Miyasu Subject: 02-17-835
Sanae-san
I can take a look on it, but no promises. Well made bugs rarely lead to any clues. Unmarked optronics and thing like that.
It it wasn't your friend, I'd look for their receiver. These things can't broadcast far due to their size and limited power. On ships, they are usually hidden between other control systems for long term surveillance - either it looks like a power supply or a circuit board. Hard to notice at first glance, but accessible to almost anyone.
They might be also collecting data in person through a handheld device, but that's not as reliable as it relies on a person to not get seen and caught. Also if your Sister is as paranoid as you say, she'd be checking personal devices of your crewmen.