It appears that this (https://www.aaroads.com/forum/proxy.php?request=http%3A%2F%2Fwww.corcohighways.org%2Fhighways%2Fgeneric%2Fmontana574.png&hash=faea5da80c33ebff9f44e8e94b053911d288aa15) is an error relic of the I-90 Frontage Road. It starts at Waldo Road and US 93 North/MT 200 West, which technically is the Frontage Road. SR 574, as Bing Maps identifies this route as, then passes through Frenchtown and ends about a 1/2 mile west of Huson.
I don't see any evidence of this route in existance in any route log that shows that there is this "SR 574". I know corco has this listed on his site.
Corco, do you know for certain if any route logs mention SR 574, or is that just put in as an error relic?
S-574 exists (but may not be signed): http://www.mdt.mt.gov/other/county_maps/MISS_CS.PDF
Page 367 of the official route log
http://www.mdt.mt.gov/publications/docs/brochures/2011_roadlog.pdf
Most of those weird little secondaries aren't signed, from what I've seen, so this is probably one of them.