Some good points made above. I'd just like to add that there are not many vacant units that do not receive occasional but regular visits for either security or maintenance purposes. These occasional visitors are likely to be employed, the vacant unit is their workplace (for the time being) and so the RR(FS)O is likely to apply.
One important point with these units is that very often the occasional visitors will unlock the way they enter but leave locks, often padlocks, on all other possible exit routes. That is, they often put themselves in long dead ends. This is not likely to be a problem if the unit is empty and the electrics are off but it could be a problem if, for example, there is an accumulation of post by the entrance they used, i.e. the only one that's unlocked, or if fire loading remains in the premises generally.
There are, of course, a number of solutions to this potential problem.