No engine check light or warning with O2 sensor unplugged
 

I inadvertently left the O2 sensor unplugged on the rear cylinder of my partners machine.

She took it for a test ride and reported the engine was some how faltering and not running smoothly.

Obviously I noticed my mistake as soon as I looked. After plugging the sensor in the engine ran fine.

The correct error code is stored that the O2 sensor is inactive.

My question is though should the engine check light come on?

It may take two run cycles (off / on / start) with the same code detected to keep the Check Engine light lit.
The engine would need to run, expire the "Warmup Enrichment" added AFR (maybe 90 seconds or so) and get to the Closed Loop minimum temperature before an O2 sensor code would set.

The Check Engine Light normally lights for 4 seconds when the ignition is turned on (key on then Run start Switch to run engine not started)

then it can then either stay off - no codes stored or detected
It can come back on after 4 seconds, for 8 seconds, then off if a code is stored "historic code".
It can stay lighted. (likely takes two run cycles to SET the light)

Based on the run I think the engine would of got to a good temperature. I wasn't about during the test run so don't know exactly what was done.

You might be correct in that the light might have been on at some point but then historic error so the light wasn't on.

Having plugged the sensor back in we went for a 215 mile ride and it ran perfectly all day.