Your mentioning that the entrance to the path may have been hidden on purpose brings to mind an incident that occurred a few years back.Should be a reasonable track depending on how it has been used. I think there was some talk about making it more fit for use but cannot remember where I read this.
There is an old line on the Black Isle between Avoch and Fortrose which is ok if you can find it. The entrance at Fortrose is well hidden and is just about wide enough for handlebars on my Brompton.
It is not signposted and my feeling is it has been hidden on purpose for whatever reason tho' it seems well used by locals.
Might be the same at St Fillans.
Our running club is adjacent to a Public School with extensive grounds. Rugby / football pitches, Rowing Club on the river etc. Theres a public footpath that meanders from one side of our running club and across part of the grounds and eventually reaches the river.The school started putting up “ Private “ signs and obstructing parts of the path. The groundskeeper would from time to time stop people and say they were trespassing.
One of our Club members was a member of the LDWA and had words with the rights of way people who investigated. The School ended up having to put in “ Public Right of Way “ signs and clear the obstructions