Doubt we’ve had oil thieves... unless they were coming round and taking a bit at a time, lol... and given the canine alarm system, I’d notice that.
But we could have a leak, yes, we’ve had one fixed already, but it’s an old boiler, old system and was empty for about 3 years before we moved in.
I mean, we could also have just used it - the heating is on a lot, the insulation is rubbish and I’m in Scotland and do not like to be cold 
We were considering swapping it for a heat pump, but we’re still working out what’s what tbh.
I might get someone to come check if there’s still a leak though.
Oh and OP, I found it was definitely worth getting loads of quotes - they varied by loads, some were about £100 for 1000l more than who we went with, some were more 