Where does sieve store its "vacation" state?

[Ron Garret]

Pigeonhole/sieve has a “vacation” feature that keeps track of which addresses it has sent messages to already so it doesn’t send too many.  The state for this feature seems to survive a dovecot restart, so it must be persistent somewhere and not just stored in RAM.  But I don’t recall configuring any place where that state would reside.  So where is it?

rg