I know your question is whether Wrentits are increasing on the valley floor, but I want to address the fire-refugees hypothesis. I don't think Wrentits are great fire escapees.
A lot of the "wrentits barely fly!" anecdotes are just that (in LA, it was a big deal when one flew across a freeway to colonize a new park), but Cornell's Birds of the World account seems to agree for the most part:
* "Most flights < 30 m; often hesitates or even avoids flights > 10 m"
*Extraordinarily-isolated occurrences mentioned in the species account are either qualified (for north of the Columbia river), represent upslope movements through habitat that's at least moderately suitable (e.g., between scrub and forest), or are un-cited (there's no reference for documented occurrences in "isolated agricultural patches in the Central Valley"--I'm guessing the Davis Wetlands occurrence would fall in that category, but there are still margins of riparian and road-ditch vegetation that could have helped it there.)
*Baker et al.(1995) reported 720 meters as the farthest distance for natal dispersal of a wrentit through suitable habitat. They said movement to another site was possible/expected, but overall, birds didn't move very far from home.
*After the 1995 Vision fire, Rich Stallcup wrote, "While it appears that most birds escaped with their lives from the forested ridges, a great many probably died in the coastal scrub of the southwest of the crest. Here wrentits, Bewick's wrens, California quail, and Nuttall's white-crowned sparrows hold year-round territories to which some would cling until time ran out. Some of these sparrows and wrentits are so sessile-by-nature that they have never strayed more than 200 yards from the spot where they were born. (Fairly poetic/anecdotal, sure, but also: Rich Stallcup)
*Finally, it wouldn't be too radical to imagine they simply can't fly very far, even if their life depends on it. There's experimental evidence showing that at least some birds from dense habitats are easily exhausted by/incapable of flights of well under 100 meters (Moore et al. 2008; they took rainforest birds onto boats and saw if they could fly over water and back to land, but many simply fell to exhaustion in the river on their flight back. Of 16 individual checker-throated antwrens/stipplethroats released from a boat 100 meters from land, none could successfully fly back; the mean distance they flew was 24 meters. Reassuringly, the birds were purportedly saved when they fell in the water). Again, not wrentits, but the phenomenon of extreme flight-limitation is certainly possible for birds of closed habitats.
So my money would be on short-distance movements or improved local recruitment, not escaping fires.