While I understand the belief that Danny doesn't have a tattoo, from a viewers standpoint it would be a lot of dramatic build-up for so little payoff if it turned out Danny just happened to be around while they where breaking windows, stealing, or beating people. I cannot imagine that is information Sonny would use to level a threat. If the writers are using the real Tanglewood Boys as a model for Sonny and his crew then the crimes were a notch above simple assault and petty theft.
Would a doctor realize the significance of a tattoo that said Tanglewood with dates underneath? Mac is a New York City cop, Hawkes is the coroner, and they didn't realize the significance of the tattoo. Rumors of gang affiliation and a criminal record are entirely different situations; a record would keep Danny off the force not teenage antics that no one suspects. In real life an actual Tanglewood Boy slightly crippled the organization by ratting out friends and enemies, these are indeed men protected from the bowels of hell. Many former gang members go on to enlist in the United States Military, because they have clean records.
As far as Danny not taking the opportunity to tell Mac the truth when questioned about the Tanglewood Boys, well that would be the whole point of keeping a secret. Danny lied because for whatever reason he feels the need to keep a secret. I cannot imagine a look of utter devastation because you were standing nearby when they beat someone up, that look was simply heart wrenching, like a child disappointing a parent. Danny truly wants Mac's respect and in Danny's young mind, gang affiliation would not go a long way towards impressing a Marine.
Personally, I think Mac suspects the truth. He is an astute man. Mac may simply be waiting for Danny to come clean.
Whatever the outcome I hope, it is a worthy successor to Tanglewood, Part 1.
Ali