"Theme=wabi" showing intermittently in the messages status area

I'm intermittently getting "theme=wabi" displayed in the "messages status" area on (among others) the welcome page -- any pointers as to where I should look to track this down? I can't replicate it reliably, but it isn't something I want popping up randomly to confuse my users! The site is in development in an XAMPP multi-site environment, and I need to fix this before I deploy in production. I'm not looking for a magic answer here, just some advice to get me started on narrowing down the cause. I'm going to try disabling all modules in turn, but that's going to be a long process because of the intermittent nature of the problem...

Running Drupal 6.5 with the Wabi theme and the following modules:

Drupal 6.5
Includes: Block, Color, Database logging, Filter, Help, Menu, Node, Path, Search, System, Taxonomy, Update status, User

GMap Module 6.x-1.0-rc1
Includes: GMap, GMap Macro Builder, GMap Taxonomy Markers

Image 6.x-1.0-alpha3
Includes: Image, Image Gallery, Image Import

Image Assist 6.x-2.x-dev (2008-Oct-15)
Includes: Image assist

insertFrame 6.x-1.2
Includes: insertFrame

Menu Breadcrumb 6.x-1.1
Includes: Menu breadcrumb

Nodeaccess 6.x-1.2
Includes: Nodeaccess

Pathauto 6.x-2.x-dev (2008-Sep-13)
Includes: Pathauto

Pathologic 6.x-1.1-beta13
Includes: Pathologic

Term Display 6.x-1.0
Includes: Term Display

Token 6.x-1.11
Includes: Token, Token actions

Views 6.x-2.0
Includes: Views, Views UI, Views exporter

Web Links 6.x-1.0
Includes: Web Links, Web Links Filter