What are the relevant vinaya guidelines if a bhikkhu was to find a likely abandoned dwelling? Suppose a bhikkhu living in the forest was to come across an abandoned shelter not frequented for several years. Dwelling is clearly in decay and not maintained, with spider webs &amp; hornet's nest etc. 

Suppose it's natutally assumed to be an illegal structure by law of the land.

Can he make use of the dwelling and appropriate the requisites if those are otherwise allowable, to do with as he sees fit? Can he dismantle the dwelling? How long does it take for requisites to be considered forsaken and fit to make use of in general?

Im looking primarily for relevant guidelines &amp; anecdotes from the vinayas &amp; commentary of early schools, not interested in mahayana/vajrayana answers.
It's not a matter of how long, since you can't know for certain anyway. If you perceive it to be abandon, then you can go ahead and use it as deem fit.