It can appear to look like hairy timber and often occurs under old terracotta roof tile battens but can be seen on other structural timber within the roof space. The photo below shows a timber purlin under a metal roof in Port Adelaide.
The lignin, which is the natural glue that holds wood fibres together, can break down due to moisture ingress, or by the chemical environment due to the air quality of the area over time, or just from proximity to the ocean.
When the fibres begin to detach the timber begins to look hairy or defibrated.
Often the hairy timber is just surface damaged and not a structural issue, I have also however in some roof areas, been able to crush the timber with my hand and pull off chunks of timber.
Chemical delignification is not a termite or pest issue, so may not be noted in a termite report, but a good building inspector should note it on a building report, if they fully inspect the ceiling space.