does milk separate?

my brain is on some sort of time delay. i couldn't remember, while at the store, whether i needed milk, but i bought some anyway on the theory that i ALWAYS need milk. as i was putting the new one behind the old (still half full) milk, i had a thought. when i got home from florida to my burgled apartment, the milk in my fridge had (naturally) gone bad. there was a layer of yellowish liquid on top of it, and at the time, i thought, "eww, it separated, i bet that smells bad" so i just tossed the whole thing into the garbage. no thought of dumping it out to recycle the plastic, because the odor of bad milk makes me heave. a week+ later, now, it occurs to me-- does milk separate? i go through so much of it that it rarely has a chance to go bad, but it seems to me that in the past, when i've bought too much, or forgot to dump it before leaving town, that it curdles, not separates. so now i'm wondering, was my burglar so pissed that he couldn't find my laptop that he pissed in my milk on his way out?? if anyone has any knowledge on this matter, please let me know, because otherwise my plan, now that i have more milk than i need, is to abandon use of the half-full jug for a week or two and use it as an experiment. and, well, yuck.