Detecting an enemy - smth I don't understand

[vorpal+5 (OP)]

I'm in a system with an enemy ship using an active sensor S196/R93. The enemy ship is 8m km away from a picket of fighters, one having a R1 active sensor with a range of 1.3m km.
I have no passive sensors in the system, no thermal or grav one.
And yet, I detect the ship. It was detected before because I had a large active sensor, but I pulled it away to another system.

So why I'm still detecting the enemy? Is it a glitch, me not understanding a rule?

[El Pip]

All ships get a small built in EM and Thermal sensor. Can't recall the details, I think it's equal to a 1 HS sensor at the tech level when you built it.

[vorpal+5 (OP)]

ah right! so it explains why I saw the 'beacon in the dark', as the enemy ship was close and emitting with her active sensor. Thanks...

[DIT_grue]

All ships get a small built in EM and Thermal sensor. Can't recall the details, I think it's equal to a 1 HS sensor at the tech level when you built it.

To be precise: the free 'look out the window' sensors each have a sensitivity of 1.

1HS is the maximum size a passive sensor is allowed to be before it is classed as a military rather than a commercial component.