Herrin Rd is a “hidden” state route. Cambria Rd might be also, and certainly is between Herrin Rd and IL 149
Wow, I see what you mean about "might be"!

On Cambria Road south of Herrin-Colp blacktop, there is an "end state maintenance" sign, and there's a similar sign heading north from Route 13. Furthermore, the road is labeled as Williamson CR-9 between the two on the official IDOT map. However, on the same map, the whole stretch is co-labeled as FAS-907—the same number as north of Herrin-Colp blacktop. The only bids I can find online that mention "FAS 907" are for the Big Muddy River bridge, which portion you've already stated is definitely a state route. So the only actual evidence I see of Cambria Road being a state route south of Herrin-Colp blacktop is that it's co-labeled as FAS-907 on the IDOT map.

IDOT's 'Getting Around Illinois' map lists the stretch between Herrin-Colp blacktop and Route 13 with 'Major Collector' as its functional class, but I'm not 100% sure that means it's not a state route. I thi-i-i-i-ink it does: I cannot identify any state route that's labeled that way (I thought I found a few, but I see no evidence of their being FAS routes now, so maybe they were turned back to counties at some point).