Um, The public did not call in the whereabouts of the brothers. The brothers panicked. They got mixed in with a robbery at a 7-11 convenient store. They made the mistake of attracting a lot of attention by killing the MIT cop. They then carjacked a motorist who managed to escape. He told a gas station operator to call in that these were the terrorists by their own admission. That's when the chase to Watertown started.
If the brothers had kept their cool, stayed out of stores getting robbed, and kept their mouths shut, and did not do reckless acts, they may still be at large.
Yes, the younger brother got caught when the ban on free movement was lifted allowing a homeowner to make the accidental discovery of the fugitive. That ban was stupid. One can only imagine the economic loss at a time when government was expending at an overtime rate. They could just as well have locked down east Watertown and spared Boston, Cambridge, Waltham, Newton, and Belmont the use of brute repression. They had a gut feeling and reasons to believe he was hiding somewhere in east Watertown. In fact he was.
How many times do we have to repeat this - there was NO robbery. The 7-11 was where the SUV's owner was able to run into and call 911.