Update: after restart,this no longer happens (should have tried that first I guess).
Today I got Galaxy watch 3, plus the companion app. That's the only difference since yesterday.
I connected it to my phone, all is good.
Now the watch is turned off.
I noticed that when I'm placing the phone down, S9, while the screen is illuminated, locked or unlocked, before touching any surface (hand, table,...) The heart rate sensor Turns on a constant red light!!!
I can reproduce this time and again just by waving a finger within 1cm.