Fenris' telepathy isn't well ranged enough to completely obiviate the use of cellphones and given the circumstances the triplets keep in constant contact with each other through text messages.
And Felix fails to send one. Fenris waits, they already agreed that wasn't enough reason to panic and in fact isn't, they get delayed all the time.
But after enough time has passed Fenris makes a detour to check on Felix personally.
An open door to a quiet apartment shouldn't be one of the scariest things Fenris has seen in his life.
But here is a door completely open, no sounds coming from the apartment inside despite their plan that they would music as a subtle signal that unfriedly Precogs wouldn't pick up on. And Felix fails to contact him back through either telepathy or telephone.
He doesn't barge in like a heroic idiot, they planned this, Fenris contacts the police, well, he mentally contacts a person in another building who was paid to keep watch.
Fenris waits at one of Felix's neighboors, a pair of nice subs.
Their person contacts the police about the suspicious circumstances at Felix's apartment.