Im playing the PC Version and it started well enough. I went to Dragon Bridge, Maro was there, and i went into the outpost, killed the guard in a sneak attack and stole the schedule. All fine, I leave and Maro is moving, its mourndas so I head to Solitude to wait and ambush him, and i seem him moving on the map and I wait till nightfall since it said he'd be resting in dour and I check the map and hes gone. Not in the castle, not on the map, and the journal option go to map is greyed out.

So I wait more, still gone, I travel to other cities and still gone. Its almost the end of the Skyrim week and no journal update and hes missing. Any ideas? I'd hope he was essential and couldn't be killed by some wild animals or dragons.

Open the console (~ key)

Type:

player.moveto 00044051

That should place you wherever he is.

If he's dead, open console, click on him, and type:

resurrect

He should start his routine again. If not you may need to "disable", then "enable" him, but try the above first.