Does someone have any ideas??

Are you measuring 3V relative to 0V.....?
The delay is the time during which it will stay in its "on" i.e triggered state, regardless.
Think "automatic garden lighting"...... which is probably what it was designed for.
If it was designed for security I would not expect a delay setting, but I don't know.
Then, probably, it will refuse to react for about 3 seconds if the "one time only" switch is set, because otherwise it could re-trigger itself if it were controlling a "hot" light.
If that is not set then it will trigger again straight away.

Which suggests that your 3V is actually "off", and that it is actually reacting to IR changes or noise all the time........ being continuously triggered by your presence.

Have you tried playing with the sensitivity (if it is like this one)...... http://qqtrading.com.my/pir-motion-sensor-module-hc-sr501

And also, thinking "automatic outside lighting" again, is it not triggering at all because there is too much ambient light? (I don't think that is the case..... I would still guess that it is being permanently triggered).
David.

Thanks for your answer and clear explanation
I measure 2.8V to to 0V.

I got a little step closer but my futile brain is not fully understanding what is going on. So I hooked up a timer (in Pd) to measure the interval time of the on/off state of the sensor. If the sensor is not detecting movement It opens for 5 seconds and if it is detecting movement it goes beyond this. If I make continuous movement the pir stays open. If there is no movement it keeps switching on/off at a regular interval.

The pir is in L mode (normal state). Of what I read is that it should go LOW when it is not triggered.

Does it somehow detect still movement (ambient light, my screen, shadows)? Or am I missing something else?

Thanks so much!