The sun is going down on an incredible day of searching unsuccessfully for Suspect 2. A guess - he was wounded in the early morning gunfight, hid out, passed out and bled out. If he is well enough hidden he will eventually be found by smell.
MUCH LATER: It's always in the last place you look, after you stop looking:
After authorities relaxed the lockdown with Tsarnaev still at large late Friday afternoon, David Henneberry headed outside and noticed that the tarp had become unsecured — something it had not done during the blizzards that hit Boston this winter. That, as boat owners know, simply will not do, so he went to fix it.
Ending the lockdown led to the discovery and capture of Suspect 2.