It's during a particularly bad coughing fit, one where Mike nearly loses his footing and comes close to falling from the side of the building he precariously hangs from, where he spots Donatello.

Not in the shadows.
Not running on a rooftop.
Just...walking through the city like it ain't no thing.

Well no wonder no one's been able to find him! He's "hiding" in plain sight!

Mike drops from his position, landing with a soft thud on alley floor.

"Donatello," he calls from the shadow.