ICEO, ICBO physical interpretation in BJT
1/ ICBO is reverse leakage current going from the Collector to the Base. This current is then amplified by β to produce additional Collector current, thus the "1+β" term. 

2/ Both currents exist simultaneously, but ICBO is included in ICEO.

3/ The reverse current ICBO is amplified, just like external Base current would be.