It was a custom Sketch I wrote to use with an Air Quality Sensor Grove. I since re-wrote it so I don't have the original source code (the new version works fine on another Bean+ I have).
Essentially, I've added a bunch of debugging info to the source code of the Grove library by Seeed (https://github.com/Seeed-Studio/Grove_Air_quality_Sensor/blob/master/examples/AirQuality_Sensor/AirQuality_Sensor.ino). What it does is, it waits for 20 seconds for the sensor to initialize and then polls the sensor constantly. Here's a log from the Serial:
[AirQuality] Initializing sensor with a delay of 20s.
[AirQuality] Sensor reading: 269
[AirQuality] Sensor ready.
[AirQuality] Polling sensor:
[AirQuality] - latest reading: 277
[AirQuality] - average reading: 269
The "[AirQuality] Starting" is at the very beginning of the void AirQuality::init(int pin) (https://github.com/Seeed-Studio/Grove_Air_quality_Sensor/blob/master/AirQuality.cpp#L42), which is called in setup().
The last non-repeating lines of the log are from the end of the AirQuality::init(int pin) and something must be happening there. In my rewrite, I've added a bunch of Serial output so, perhaps, it is to blame.
Another thing is, when the Bean+ is switched on, based in the trial output, the sketch is not being run automatically, I need to send the "Start sketch" command to do so.