It's called a bond revocation hearing. The government produces what's called a motion to revoke bond. The defendant and his attorney can plead that the allegations are true or not true.
If the judge find probable cause that the bond conditions have been revoked, the defendant is taken into custody.
Additionally, if he has a probation hearing tomorrow and doesn't attend, his PO can report this to the prosecution and the government will file a motion to revoke.
He may be arrested on the spot by his PO (at his PO's discretion), have his PO report it to the prosecution, and then the government would issue a motion to revoke.
But there's no "fail a drug test and then the clock is ticking for your surprise capture at a time and place of our choosing," as many of you seem to think, for no reason at all.
How about you just show me the warrant you saw, or stop lying and just admit you haven't seen it but you believe it exists?