Sorry PeterT, you're right, I was scanning another device
I was betrayed from the way Kobo manages wifi connections: you can have wifi on, but you're not connected to your default network until you open your browser. So, fortunately, no ssh backdoor, and I learnt something new about my 1-day old Kobo
By the way, this raises another question: do you know which command or script Kobo uses to bring up the default wifi network?