CIM is just Cum in Mouth, no matter how you like it, trough a BJ, jerking yourself in her mouth, whatever... Agencies list it to mention its ok with a particular girl.
As for cumming without notice, well either your a silent one or the girl is not very aware, when im about to cum the girl can see it cumming, both my face or how i make my body solid hard... i dunno... Im just super resistant these days... its crazy... I mean the first 2-3 time i saw escorts i was able to cum in there mouth in no time, not much effort. Now she got to go double time and i could resist cuming for the whole hour if i want it. I even stop masturbating 2 days before a meeting just to be sure i will be at max pleasure...
And i don't think it comes from the look of the girl, as most i saw recently where very great looking. Maybe BBJ skills can be a factor... i guess it is sure, but either i saw just so many hot girls the effect is not the same anymore?