In T181655#2615355, @Falkenheim wrote:

Its not about nigh time but in general, new patch broke the zombies again, they are way too aware now, sneaking on them to perform a stealth kill is almost impossible if it's not raining. They spot you throught walls and sees you from way too far.

You can see this beeing discussed on RD https://www.reddit.com/r/dayz/comments/1d3zlpl/zombies_are_broken_now/