If the OIC has a planned holiday - what do you want them to do - cancel it. They are entitled to leave just as much as anyone else.
If they work shifts, say 4 on 4 off, then you take 4 days leave, but get the 8 days off either side of it. so taking 4 days off actual works out as 12 days away from work
So it quickly adds up - especially if the OIC is perhaps on a course etc when they return; so they will be away from operational duties for a while.
Not sure what the evidence trail is - but if they are looking at downloads of phones, computers ect - they can take a while to come back.