In reply to tlbno1:
Maybe it was open by the estate on that occasion in July?? Did you read his tale - not so sure about this last paragraph as he came off Schiehallion : "
The first steep bit was quite easy as it was soft short heather. I continues to the observatory site and placed a marker cross of toilet roll held down with stones. The reason was I wanted to be able to identify the site from the road and there was nothing of note to see seen. The thin paper would soon deteriorate when the rains came. Once down to the road I could see the cross through the binoculars. It was then an easy walk back to the car park where most of the cars had left." Nice touch.
This photo (from 2012) suggests "This bothy has been closed due to vandalism in recent years, and was locked on this occasion."
https://www.geograph.org.uk/photo/2871748