How are missing alerts handled?
It is important to note that your partnership agreements of which the opponents must be made aware specifically includes all inferences from repeated occurrences. So yes, if you make the same unusual action twice in a short span of time the burden of proof is on you to establish that the action is not a partnership agreement.
From Laws of Duplicate Bridge - 2008 Revised Authorized Edition

Law 40
Partnership Agreements
A. Player's Systemic Agreements
1 (a): Partnership understandings as to the methods adopted by the partnership may be reached explicitly in discussion or implicitly through
mutual experience or awareness of the players.